ac/llvm: Drop memory_barrier_buffer impl

Both radeonsi and radv use scoped barriers, so this should not be possible to
hit.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23191>

src/amd/llvm/ac_nir_to_llvm.c

@@ -3282,9 +3282,6 @@ static bool visit_intrinsic(struct ac_nir_context *ctx, nir_intrinsic_instr *ins
    case nir_intrinsic_demote_if:
       emit_demote(ctx, instr);
       break;
-   case nir_intrinsic_memory_barrier_buffer:
-      ac_build_waitcnt(&ctx->ac, AC_WAIT_VLOAD | AC_WAIT_VSTORE);
-      break;
    case nir_intrinsic_scoped_barrier: {
       assert(!(nir_intrinsic_memory_semantics(instr) &
                (NIR_MEMORY_MAKE_AVAILABLE | NIR_MEMORY_MAKE_VISIBLE)));