MNT: remove mpl_toolkits.axes_grid

Author: oscargus

doc/api/next_api_changes/removals/22886-OG.rst

@@ -0,0 +1,7 @@
+``mpl_toolkits.axes_grid`` module removed
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+All functionally from ``mpl_toolkits.axes_grid`` can be found in either
+`mpl_toolkits.axes_grid1` or `mpl_toolkits.axisartist`. Axes classes
+from ``mpl_toolkits.axes_grid`` based on ``Axis`` from
+`mpl_toolkits.axisartist` can be found in `mpl_toolkits.axisartist`.