Remove reg_universe method from MachBackend and MachInst

cranelift/codegen/src/isa/aarch64/inst/mod.rs

@@ -2017,10 +2017,6 @@ impl MachInst for Inst {
         }
     }
 
-    fn reg_universe(flags: &settings::Flags) -> RealRegUniverse {
-        create_reg_universe(flags)
-    }
-
     fn worst_case_size() -> CodeOffset {
         // The maximum size, in bytes, of any `Inst`'s emitted code. We have at least one case of
         // an 8-instruction sequence (saturating int-to-float conversions) with three embedded