refactor: revert redundant change

Author: DKER2

Diff for: sdks/python/apache_beam/transforms/core.py

@@ -1504,8 +1504,8 @@ def _check_fn_use_yield_and_return(fn):
     has_yield = False
     has_return = False
     return_none_warning = (
-        f"No iterator is returned by the "
-        f"process method in {fn.__self__.__class__}.")
+        "No iterator is returned by the process method in %s.",
+        fn.__self__.__class__)
     for line in source_code.split("\n"):
       lstripped_line = line.lstrip()
       if lstripped_line.startswith("yield ") or lstripped_line.startswith(