Remove the old x86 backend

cranelift/codegen/src/isa/legacy/mod.rs

@@ -1,12 +1,4 @@
 //! Legacy ("old-style") backends that will be removed in the future.
 
-// N.B.: the old x86-64 backend (`x86`) and the new one (`x64`) are both
-// included whenever building with x86 support. The new backend is the default,
-// but the old can be requested with `BackendVariant::Legacy`. However, if this
-// crate is built with the `old-x86-backend` feature, then the old backend is
-// default instead.
-#[cfg(feature = "x86")]
-pub(crate) mod x86;
-
 #[cfg(feature = "riscv")]
 pub(crate) mod riscv;