Deprecate AxesDivider.append_axes(..., add_to_figure=True).

This was never ever used in the git history, has a simple alternative,
and is consistent with 3b4d7c4 which removed the similar `add_all` from
other axes_grid APIs.

Author: anntzer

doc/api/next_api_changes/deprecations/20065-AL.rst

@@ -0,0 +1,4 @@
+``AxesDivider.append_axes(..., add_to_figure=False)``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+... is deprecated.  Use ``ax.remove()`` to remove the Axes from the figure if
+needed.

lib/mpl_toolkits/axes_grid1/axes_divider.py

@@ -525,6 +525,7 @@ def new_vertical(self, size, pad=None, pack_start=False, **kwargs):
         ax.set_axes_locator(locator)
         return ax
 
+    @_api.delete_parameter("3.5", "add_to_figure", alternative="ax.remove()")
     def append_axes(self, position, size, pad=None, add_to_figure=True,
                     **kwargs):
         """