Remove an out-of-date comment.

The part about zero-sized structures is totally wrong. The rest of
it has almost no explanatory value; there are better explanations in
comments elsewhere.

compiler/rustc_mir_dataflow/src/impls/mod.rs

@@ -1,7 +1,3 @@
-//! Dataflow analyses are built upon some interpretation of the
-//! bitvectors attached to each basic block, represented via a
-//! zero-sized structure.
-
 mod borrowed_locals;
 mod initialized;
 mod liveness;