Remove the old x86 backend

cranelift/filetests/filetests/simple_preopt/simplify_instruction_into_alias_of_value.clif

@@ -1,18 +0,0 @@
-test simple_preopt
-target aarch64
-target x86_64
-
-;; The `isub` is a no-op, but we can't replace the whole `isub` instruction with
-;; its `v2` operand's instruction because `v2` is one of many results. Instead,
-;; we need to make an alias `v3 -> v2`.
-
-function %replace_inst_with_alias() -> i32 {
-block0:
-    v0 = iconst.i32 0
-    v1, v2 = x86_smulx v0, v0
-    v3 = isub v2, v0
-    ; check:  v0 = iconst.i32 0
-    ; nextln: v1, v2 = x86_smulx v0, v0
-    ; nextln: v3 -> v2
-    return v3
-}