Remove rustc_allow_const_fn_ptr

This was a hack to work around the lack of an escape hatch for the "min
`const fn`" checks in const-stable functions. Now that we have co-opted
`allow_internal_unstable` for this purpose, we no longer need the
bespoke attribute.

compiler/rustc_span/src/symbol.rs

@@ -885,7 +885,6 @@ symbols! {
         rustc,
         rustc_allocator,
         rustc_allocator_nounwind,
-        rustc_allow_const_fn_ptr,
         rustc_args_required_const,
         rustc_attrs,
         rustc_builtin_macro,