Discard instead of remove members of only_once_messages

Prior to this commit, messages would sometimes not be present in
only_once_messages, causing an error. This uses the discard method which
won't error if the message isn't present.

mypy/errors.py

@@ -676,7 +676,7 @@ def clear_errors_in_targets(self, path: str, targets: set[str]) -> None:
                     new_errors.append(info)
                     has_blocker |= info.blocker
                 elif info.only_once:
-                    self.only_once_messages.remove(info.message)
+                    self.only_once_messages.discard(info.message)
             self.error_info_map[path] = new_errors
             if not has_blocker and path in self.has_blockers:
                 self.has_blockers.remove(path)