Fix ForkedConvRepo add_injector signature

src/breakshaft/convertor.py

@@ -194,9 +194,6 @@ class ForkedConvRepo(ConvRepo):
         self._convertor_set = fork_with
         self._base_repo = fork_from
 
-    def add_injector(self, func: Callable, rettype: Optional[type] = None):
-        self._convertor_set |= set(ConversionPoint.from_fn(func, rettype=rettype))
-
     @property
     def convertor_set(self):
         return self._base_repo.convertor_set | self._convertor_set