Remove FIXME about NLL diagnostic that is already improved

The FIXME was added in 46984 when the diagnostic message looked like
this:

    // FIXME(#46983): error message should be better
    &s.0 //~ ERROR free region `` does not outlive free region `'static`

The message was improved in 90667 and now looks like this:

    &s.0 //~ ERROR lifetime may not live long enough

but the FIXME was not removed. The issue 46983 about that diagnostics
should be improved has been closed. We can remove the FIXME now.

tests/ui/nll/guarantor-issue-46974.rs

@@ -9,7 +9,6 @@ fn foo(s: &mut (i32,)) -> i32 {
 }
 
 fn bar(s: &Box<(i32,)>) -> &'static i32 {
-    // FIXME(#46983): error message should be better
     &s.0 //~ ERROR lifetime may not live long enough
 }
 

tests/ui/nll/guarantor-issue-46974.stderr

@@ -10,11 +10,10 @@ LL |     *x
    |     -- borrow later used here
 
 error: lifetime may not live long enough
-  --> $DIR/guarantor-issue-46974.rs:13:5
+  --> $DIR/guarantor-issue-46974.rs:12:5
    |
 LL | fn bar(s: &Box<(i32,)>) -> &'static i32 {
    |           - let's call the lifetime of this reference `'1`
-LL |     // FIXME(#46983): error message should be better
 LL |     &s.0
    |     ^^^^ returning this value requires that `'1` must outlive `'static`