rustc_borrowck: suggest_ampmut(): Inline unneeded local var

Since the previos commit renamed `assignment_rhs_span` to just
`rhs_span` there is no need for a variable just to shorten the
expression on the next line. Inline the variable.

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -1500,9 +1500,8 @@ fn suggest_ampmut<'tcx>(
         // if the reference is already mutable then there is nothing we can do
         // here.
         if !is_mut {
-            let span = rhs_span;
             // shrink the span to just after the `&` in `&variable`
-            let span = span.with_lo(span.lo() + BytePos(1)).shrink_to_lo();
+            let span = rhs_span.with_lo(rhs_span.lo() + BytePos(1)).shrink_to_lo();
 
             // FIXME(Ezrashaw): returning is bad because we still might want to
             // update the annotated type, see #106857.