CLN: Remove Categorical.itemsize (#46137)

* CLN: Remove Categorical.itemsize

* fix CI error

Co-authored-by: Terji Petersen <terjipetersen@Terjis-Air.fritz.box>

Author: topper-123

pandas/core/arrays/categorical.py

@@ -43,7 +43,6 @@
 )
 from pandas.compat.numpy import function as nv
 from pandas.util._decorators import (
-    cache_readonly,
     deprecate_kwarg,
     deprecate_nonkeyword_arguments,
 )
@@ -563,13 +562,6 @@ def astype(self, dtype: AstypeArg, copy: bool = True) -> ArrayLike:
 
         return result
 
-    @cache_readonly
-    def itemsize(self) -> int:
-        """
-        return the size of a single category
-        """
-        return self.categories.itemsize
-
     def to_list(self):
         """
         Alias for tolist.