Remove MachBackend

It is identical to TargetIsa

cranelift/codegen/src/isa/mod.rs

@@ -49,7 +49,7 @@ use crate::flowgraph;
 use crate::ir::{self, Function};
 #[cfg(feature = "unwind")]
 use crate::isa::unwind::systemv::RegisterMappingError;
-use crate::machinst::{MachBackend, MachCompileResult, TextSectionBuilder, UnwindInfoKind};
+use crate::machinst::{MachCompileResult, TextSectionBuilder, UnwindInfoKind};
 use crate::settings;
 use crate::settings::SetResult;
 use crate::CodegenResult;
@@ -273,9 +273,6 @@ pub trait TargetIsa: fmt::Display + Send + Sync {
     /// will be "labeled" or might have calls between them, typically the number
     /// of defined functions in the object file.
     fn text_section_builder(&self, num_labeled_funcs: u32) -> Box<dyn TextSectionBuilder>;
-
-    /// Get the new-style MachBackend.
-    fn get_mach_backend(&self) -> &dyn MachBackend;
 }
 
 /// Methods implemented for free for target ISA!