adapter: Delete migration for avg_internal_v1 (#21848)

Removes the migration introduced as part of
#21219

### Motivation

* This PR refactors existing code.
  Cleans up a migration I've been meaning to delete

### Checklist

- [ ] This PR has adequate test coverage / QA involvement has been duly
considered.
- [ ] This PR has an associated up-to-date [design
doc](https://github.com/MaterializeInc/materialize/blob/main/doc/developer/design/README.md),
is a design doc
([template](https://github.com/MaterializeInc/materialize/blob/main/doc/developer/design/00000000_template.md)),
or is sufficiently small to not require a design.
  <!-- Reference the design in the description. -->
- [ ] If this PR evolves [an existing `$T ⇔ Proto$T`
mapping](https://github.com/MaterializeInc/materialize/blob/main/doc/developer/command-and-response-binary-encoding.md)
(possibly in a backwards-incompatible way), then it is tagged with a
`T-proto` label.
- [ ] If this PR will require changes to cloud orchestration or tests,
there is a companion cloud PR to account for those changes that is
tagged with the release-blocker label
([example](MaterializeInc/cloud#5021)).
<!-- Ask in #team-cloud on Slack if you need help preparing the cloud
PR. -->
- [x] This PR includes the following [user-facing behavior
changes](https://github.com/MaterializeInc/materialize/blob/main/doc/developer/guide-changes.md#what-changes-require-a-release-note):
  - N/a

src/adapter/src/catalog/migrate.rs

@@ -15,18 +15,12 @@ use mz_ore::collections::CollectionExt;
 use mz_ore::now::EpochMillis;
 use mz_sql::ast::display::AstDisplay;
 use mz_sql::ast::Raw;
-use mz_sql_parser::ast::visit_mut::VisitMut;
-use mz_sql_parser::ast::{Function, Ident};
 use mz_storage_types::connections::ConnectionContext;
 use semver::Version;
 use tracing::info;
 
 use crate::catalog::{Catalog, ConnCatalog};
 
-/// A flag to indicate whether or not we've already run the migration to rename existing uses of
-/// the AVG(...) function. Runs in versions <=0.66.
-const RENAME_AVG_FUNCTION_MIGRATION_FLAG: &str = "rename_avg_function_migration_flag";
-
 async fn rewrite_items<F>(
     tx: &mut Transaction<'_>,
     cat: Option<&ConnCatalog<'_>>,
@@ -72,21 +66,6 @@ pub(crate) async fn migrate(
     // First, do basic AST -> AST transformations.
     // rewrite_items(&mut tx, None, |_tx, _cat, _stmt| Box::pin(async { Ok(()) })).await?;
 
-    // Only run this migration if we haven't run it already.
-    let avg_function_has_run =
-        tx.check_migration_has_run(RENAME_AVG_FUNCTION_MIGRATION_FLAG.to_string())?;
-    info!("{RENAME_AVG_FUNCTION_MIGRATION_FLAG}, already_run: {avg_function_has_run}");
-
-    if !avg_function_has_run {
-        rewrite_items(&mut tx, None, |_tx, _cat, stmt| {
-            Box::pin(async { ast_rename_avg_function_0_66_0(stmt) })
-        })
-        .await?;
-        // Important: Mark the migration as being completed in the same transaction that we used
-        // to do the migration.
-        tx.mark_migration_has_run(RENAME_AVG_FUNCTION_MIGRATION_FLAG.to_string())?;
-    }
-
     // Then, load up a temporary catalog with the rewritten items, and perform
     // some transformations that require introspecting the catalog. These
     // migrations are *weird*: they're rewriting the catalog while looking at
@@ -131,48 +110,6 @@ pub(crate) async fn migrate(
 // AST migrations -- Basic AST -> AST transformations
 // ****************************************************************************
 
-fn ast_rename_avg_function_0_66_0(
-    stmt: &mut mz_sql::ast::Statement<Raw>,
-) -> Result<(), anyhow::Error> {
-    struct AvgFunctionRenamer;
-    impl<'ast> VisitMut<'ast, Raw> for AvgFunctionRenamer {
-        fn visit_function_mut(&mut self, node: &'ast mut Function<Raw>) {
-            let components = &mut node.name.name_mut().0;
-            let len = components.len();
-            if len < 2 {
-                return;
-            }
-            let Some(last_two) = components.get_mut(len - 2..) else {
-                return;
-            };
-
-            // Rewrite the the function to be "avg_internal_v1".
-            if last_two[0].as_str() == "pg_catalog" && last_two[1].as_str() == "avg" {
-                last_two[0] = Ident::new("mz_catalog");
-                last_two[1] = Ident::new("avg_internal_v1");
-            }
-
-            // No one should have an object with this function as a dependency, but we check for it
-            // here just to be safe.
-            if last_two[0].as_str() == "mz_internal" && last_two[1].as_str() == "mz_avg_promotion" {
-                last_two[1] = Ident::new("mz_avg_promotion_internal_v1");
-            }
-
-            // Continue to recurse.
-            self.visit_function_args_mut(&mut node.args);
-            if let Some(v) = &mut node.filter {
-                self.visit_expr_mut(&mut *v);
-            }
-            if let Some(v) = &mut node.over {
-                self.visit_window_spec_mut(&mut *v);
-            }
-        }
-    }
-
-    AvgFunctionRenamer.visit_statement_mut(stmt);
-    Ok(())
-}
-
 // ****************************************************************************
 // Semantic migrations -- Weird migrations that require access to the catalog
 // ****************************************************************************