remove undocumented isnull / fillnull

Author: MarcoGorelli

pandas-stubs/core/indexes/base.pyi

@@ -418,10 +418,8 @@ class Index(IndexOpsMixin[S1], ElementOpsMixin[S1]):
     def hasnans(self) -> bool: ...
     @final
     def isna(self) -> Index[bool]: ...
-    isnull = isna
     @final
     def notna(self) -> Index[bool]: ...
-    notnull = notna
     def fillna(self, value: Scalar) -> Index: ...
     def dropna(self, how: AnyAll = "any") -> Self: ...
     def unique(self, level: Hashable | None = None) -> Self: ...