CLN: Remove downcast keyword in MultiIndex.fillna (#58923)

Author: mroeschke

pandas/core/indexes/multi.py

@@ -1677,7 +1677,7 @@ def duplicated(self, keep: DropKeep = "first") -> npt.NDArray[np.bool_]:
     # (previously declared in base class "IndexOpsMixin")
     _duplicated = duplicated  # type: ignore[misc]
 
-    def fillna(self, value, downcast=None):
+    def fillna(self, value):
         """
         fillna is not implemented for MultiIndex
         """