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.
This variable is used later in this function