Remove remaining obsolete compatibility options

Gnome-panel 2.32 contained code to upgrade from very old versions. These old versions have no corresponding MATE version, so the code doesn't do anything and is never called. Most of it is already removed, but this pull request removes the remainder.

Code taken from commits b9fc819b956319b6ae99fa13bf54c4f54d9e3a0e and e57b3afd9255671789ea6c1b6136bbad82573cdc of gnome-panel.
The commit changed the only use of the variable to PANEL_OBJECT_ACTION, since it was the only value the variable could have. I forgot to change that line.