RBTools 5.1 introduced `Patcher` classes for applying patches, replacing the `BaseSCMClient.apply_patch()` and `apply_patch_for_empty_files()` methods. SCMClients that only overrode `apply_patch()` were wrapped in a `_LegacyPatcher`, along with a deprecation warning. This removes `_LegacyPatcher`, the compatibility branch in `get_patcher()`, and the now-unreachable `apply_patch_for_empty_files()` stub on `BaseSCMClient`. SCMClients must now set `patcher_cls`. `apply_patch()` has been left as a convenience wrapper. Testing Done: Ran unit tests.