remove const_slice_index annotations, it never had a feature gate anyway

tests/ui/consts/const-eval/ub-slice-get-unchecked.rs

@@ -1,7 +1,5 @@
 //@ known-bug: #110395
 
-#![feature(const_slice_index)]
-
 const A: [(); 5] = [(), (), (), (), ()];
 
 // Since the indexing is on a ZST, the addresses are all fine,

tests/ui/consts/const-eval/ub-slice-get-unchecked.stderr

@@ -1,5 +1,5 @@
 error[E0015]: cannot call non-const fn `core::slice::<impl [()]>::get_unchecked::<std::ops::Range<usize>>` in constants
-  --> $DIR/ub-slice-get-unchecked.rs:9:29
+  --> $DIR/ub-slice-get-unchecked.rs:7:29
    |
 LL | const B: &[()] = unsafe { A.get_unchecked(3..1) };
    |                             ^^^^^^^^^^^^^^^^^^^