remove tests/ui/pattern/no_ref_mut_behind_and.rs

This was a test for `ref_pat_eat_one_layer_2024` downgrading a `mut ref`
default binding mode to `ref` within a shared reference pattern (i.e.
Rule 3) on edition 2021 specifically. Since it's near-identical to
another existing test (currently in `ref_pat_eat_one_layer_2021.rs` in
the "experimental" rfc 3627 subdirectory) and that particular feature
gate's typing rulesets are planned to no longer have Rule 3, I'm opting
to remove it rather than continue maintaining it separately.

tests/ui/pattern/no_ref_mut_behind_and.rs

@@ -1,9 +0,0 @@
-//@ edition: 2021
-//@ run-pass
-#![allow(incomplete_features)]
-#![feature(ref_pat_eat_one_layer_2024)]
-
-fn main() {
-    let &[[x]] = &[&mut [42]];
-    let _: &i32 = x;
-}