Remove accidentally added code lines at wrong place (graphql-python#263)

Interestingly, they appear as syntax error with mypy
in Python 3.8, but not in Python 3.7.
We should officially support Python 3.8 when gevent 1.5
is available (currently Python 3.8 is not yet supported).

graphql/validation/rules/overlapping_fields_can_be_merged.py

@@ -341,9 +341,6 @@ def _collect_conflicts_between_fragments(
             fragment_name2,
         )
 
-    conflicts,  # type: List[Tuple[Tuple[str, str], List[Node], List[Node]]]
-    are_mutually_exclusive,  # type: bool
-
 
 def _find_conflicts_between_sub_selection_sets(
     context,  # type: ValidationContext