Mark the pushpop_unsafe feature as "removed"

This marks the pushpop_unsafe feature as removed inside the feature_gate.
It was added in commit 1829fa5199 and then
removed again in commit d399098fd8 .
Seems that the second commit forgot to mark it as removed in feature_gate.rs.

This enables us to remove another element from the whitelist of non gate
tested unstable lang features (issue #39059).

src/libsyntax/ext/expand.rs

@@ -926,7 +926,6 @@ impl<'feat> ExpansionConfig<'feat> {
         fn enable_trace_macros = trace_macros,
         fn enable_allow_internal_unstable = allow_internal_unstable,
         fn enable_custom_derive = custom_derive,
-        fn enable_pushpop_unsafe = pushpop_unsafe,
     }
 }