refactor: remove unused field

PR #3131 fixed the failing builds by allowing this field to be dead.
After looking at it further the field is not being used and can be
removedi completely.

cranelift/codegen/meta/src/isa/x86/mod.rs

@@ -78,7 +78,6 @@ pub(crate) fn define(shared_defs: &mut SharedDefinitions) -> TargetIsa {
 
     TargetIsa::new(
         "x86",
-        inst_group,
         settings,
         regs,
         recipes,