remove what seems to be an outdated comment

Even in the PR that introduced this comment, it does not seem like these locals are actually ignored -- just their `source_info` is adjusted:
https://github.com/rust-lang/rust/pull/44700/files#diff-ae2f3c7e2f9744f7ef43e96072b10e98d4e3fe74a3a399a3ad8a810fbe56c520R139

Author: RalfJung

compiler/rustc_middle/src/mir/mod.rs

@@ -821,9 +821,6 @@ pub struct LocalDecl<'tcx> {
     /// flag drop flags to avoid triggering this check as they are introduced
     /// after typeck.
     ///
-    /// Unsafety checking will also ignore dereferences of these locals,
-    /// so they can be used for raw pointers only used in a desugaring.
-    ///
     /// This should be sound because the drop flags are fully algebraic, and
     /// therefore don't affect the OIBIT or outlives properties of the
     /// generator.