Remove redundant check in IndexVariable (#8320)

* IndexVariable can only  have pandasindexadapter

* Update variable.py

Author: Illviljan

xarray/core/variable.py

@@ -2787,13 +2787,7 @@ def copy(self, deep: bool = True, data: T_DuckArray | ArrayLike | None = None):
             data copied from original.
         """
         if data is None:
-            data_old = self._data
-
-            if not isinstance(data_old, indexing.MemoryCachedArray):
-                ndata = data_old
-            else:
-                # don't share caching between copies
-                ndata = indexing.MemoryCachedArray(data_old.array)
+            ndata = self._data
 
             if deep:
                 ndata = copy.deepcopy(ndata, None)