Remove the old x86 backend

cranelift/codegen/src/machinst/adapter.rs

@@ -3,7 +3,7 @@
 use crate::binemit;
 use crate::ir;
 use crate::isa::{
-    BackendVariant, EncInfo, Encoding, Encodings, Legalize, RegClass, RegInfo, TargetIsa,
+    EncInfo, Encoding, Encodings, Legalize, RegClass, RegInfo, TargetIsa,
 };
 use crate::machinst::*;
 use crate::regalloc::RegisterSet;
@@ -64,10 +64,6 @@ impl TargetIsa for TargetIsaAdapter {
         self.backend.isa_flags()
     }
 
-    fn variant(&self) -> BackendVariant {
-        BackendVariant::MachInst
-    }
-
     fn hash_all_flags(&self, hasher: &mut dyn Hasher) {
         self.backend.hash_all_flags(hasher);
     }