Remove dead code from compare_const_vals.

It's never executed when running the entire test suite. I think it's
because of the early return at the top of the function if `a.ty() != ty`
succeeds.

Author: nnethercote

compiler/rustc_mir_build/src/thir/pattern/mod.rs

@@ -15,7 +15,6 @@ use rustc_hir::def::{CtorOf, DefKind, Res};
 use rustc_hir::pat_util::EnumerateAndAdjustIterator;
 use rustc_hir::RangeEnd;
 use rustc_index::vec::Idx;
-use rustc_middle::mir::interpret::{get_slice_bytes, ConstValue};
 use rustc_middle::mir::interpret::{ErrorHandled, LitToConstError, LitToConstInput};
 use rustc_middle::mir::{self, UserTypeProjection};
 use rustc_middle::mir::{BorrowKind, Field, Mutability};
@@ -795,15 +794,5 @@ pub(crate) fn compare_const_vals<'tcx>(
         };
     }
 
-    if let ty::Str = ty.kind() && let (
-        Some(a_val @ ConstValue::Slice { .. }),
-        Some(b_val @ ConstValue::Slice { .. }),
-    ) = (a.try_to_value(tcx), b.try_to_value(tcx))
-    {
-        let a_bytes = get_slice_bytes(&tcx, a_val);
-        let b_bytes = get_slice_bytes(&tcx, b_val);
-        return from_bool(a_bytes == b_bytes);
-    }
-
     fallback()
 }