CLN: removed the 'diff' method for Index

Deprecated all the way back in `0.15.0` <a
href="https://github.com/pydata/pandas/pull/8227">here</a>.

Author: gfyoung <gfyoung17@gmail.com>

Closes #13669 from gfyoung/remove-index-diff and squashes the following commits:

7dca659 [gfyoung] CLN: removed the 'diff' method for Index

Author: gfyoung

doc/source/whatsnew/v0.19.0.txt

@@ -602,6 +602,7 @@ Removal of prior version deprecations/changes
 - ``pd.Categorical`` has dropped setting of the ``ordered`` attribute directly in favor of the ``set_ordered`` method (:issue:`13671`)
 - ``pd.Categorical`` has dropped the ``levels`` attribute in favour of ``categories`` (:issue:`8376`)
 - ``DataFrame.to_sql()`` has dropped the ``mysql`` option for the ``flavor`` parameter (:issue:`13611`)
+- ``pd.Index`` has dropped the ``diff`` method in favour of ``difference`` (:issue:`13669`)
 
 - Removal of the legacy time rules (offset aliases), deprecated since 0.17.0 (this has been alias since 0.8.0) (:issue:`13590`)
 

pandas/indexes/base.py

@@ -1965,8 +1965,6 @@ def difference(self, other):
 
         return this._shallow_copy(the_diff, name=result_name)
 
-    diff = deprecate('diff', difference)
-
     def symmetric_difference(self, other, result_name=None):
         """
         Compute the symmetric difference of two Index objects.