[lldb] Delete RewriteObjCClassReferences (NFC)

The `RewriteObjCClassReferences` pass was applicable only to the code generated for the
fragile ObjC ABI (v1). That ABI is no longer active (last used for i386 macOS), which
means this pass has no effect.

Sources: `OBJC_CLASS_REFERENCES_` is emitted only by `CGObjCMac`, and not by
`CGObjCNonFragileABIMac`.

Differential Revision: https://reviews.llvm.org/D153802