drop deprecated value if-available for download-ci-llvm option

It's been 5 months since we deprecated this. It should be fine to drop its support now.

Signed-off-by: onur-ozkan <work@onurozkan.dev>

src/bootstrap/src/core/config/config.rs

@@ -2488,11 +2488,6 @@ impl Config {
 
                 b
             }
-            // FIXME: "if-available" is deprecated. Remove this block later (around mid 2024)
-            // to not break builds between the recent-to-old checkouts.
-            Some(StringOrBool::String(s)) if s == "if-available" => {
-                llvm::is_ci_llvm_available(self, asserts)
-            }
             Some(StringOrBool::String(s)) if s == "if-unchanged" => if_unchanged(),
             Some(StringOrBool::String(other)) => {
                 panic!("unrecognized option for download-ci-llvm: {:?}", other)