Remove some unnecessary FIXME comments.

The comments didn't make much sense to me. I asked Matthew Jasper on
Zulip about it and they said:

> I think that at the time I wanted to replace all (or most of) this
> with a reference to the HIR Id of the variable. I'll give this a look
> to see if it's still a reasonable idea, but removing the comments is
> fine.

and then:

> I don't think that changing this to an HirId would be better,
> recovering the information from the HIR seems like too much effort in
> exchange for making the MIR a little smaller.

Author: nnethercote

compiler/rustc_middle/src/mir/mod.rs

@@ -961,7 +961,6 @@ pub struct LocalDecl<'tcx> {
     /// Temporaries and the return place are always mutable.
     pub mutability: Mutability,
 
-    // FIXME(matthewjasper) Don't store in this in `Body`
     pub local_info: ClearCrossCrate<Box<LocalInfo<'tcx>>>,
 
     /// The type of this local.
@@ -971,7 +970,6 @@ pub struct LocalDecl<'tcx> {
     /// e.g., via `let x: T`, then we carry that type here. The MIR
     /// borrow checker needs this information since it can affect
     /// region inference.
-    // FIXME(matthewjasper) Don't store in this in `Body`
     pub user_ty: Option<Box<UserTypeProjections>>,
 
     /// The *syntactic* (i.e., not visibility) source scope the local is defined
@@ -1079,7 +1077,6 @@ pub enum LocalInfo<'tcx> {
     AggregateTemp,
     /// A temporary created for evaluation of some subexpression of some block's tail expression
     /// (with no intervening statement context).
-    // FIXME(matthewjasper) Don't store in this in `Body`
     BlockTailTemp(BlockTailInfo),
     /// A temporary created during evaluating `if` predicate, possibly for pattern matching for `let`s,
     /// and subject to Edition 2024 temporary lifetime rules