remove mate-panelrc

this is a deprecation file which was also deprecated by gnome in 2011.
https://git.gnome.org/browse/gnome-panel/commit/?id=09fa68b0b4d9ae185ad0ac703cbaafe9eb052124
I did test this example file and copied it to ~.matepanel.rc and add a content. Iit doesn't work. After rename it to  .gtkrc-2.0 it works.
So i quess the method to change GTK setting only for the panel is obsolete, imo.