Remove reloc_constant

It is no longer used by the new backends

crates/cranelift/src/obj.rs

@@ -310,16 +310,6 @@ impl<'a> ObjectBuilder<'a> {
                 Reloc::Abs4 => (RelocationKind::Absolute, RelocationEncoding::Generic, 32),
                 Reloc::Abs8 => (RelocationKind::Absolute, RelocationEncoding::Generic, 64),
 
-                // This is emitted by the old x86 backend and is only present
-                // for when the constant rodata is separated from the code
-                // itself. We don't do that, though, so we ignore these
-                // relocations since the offsets already listed here are already
-                // correct.
-                //
-                // FIXME(#3009): when the old backend is removed delete this
-                // case.
-                Reloc::X86PCRelRodata4 => continue,
-
                 other => unimplemented!("Unimplemented relocation {:?}", other),
             };
             self.obj