CLN: remove deprecated index.format (#1576)

Author: MarcoGorelli

pandas-stubs/core/indexes/base.pyi

@@ -414,12 +414,6 @@ class Index(IndexOpsMixin[S1], ElementOpsMixin[S1]):
         self, repeats: int | AnyArrayLikeInt | Sequence[int], axis: None = None
     ) -> Self: ...
     def copy(self, name: Hashable = ..., deep: bool = False) -> Self: ...
-    def format(
-        self,
-        name: bool = ...,
-        formatter: Callable[..., Any] | None = ...,
-        na_rep: _str = ...,
-    ) -> list[_str]: ...
     def to_series(
         self, index: Index | None = None, name: Hashable | None = None
     ) -> Series[S1]: ...