Remove Metacity support from libsteticui

There are two main reasons for this:

* Currently, the Metacity support code is not working because it is reading theme information from gconf, while Metacity switched from gconf to GSettings in 2.34.2.
* The latest Metacity version (3.12) is now using Gtk+ 3, not Gtk+ 2. However, mixing Gtk+ 2 and Gtk +3 code in the same process will lead to a crash.

See [Metacity NEWS file](https://git.gnome.org/browse/metacity/tree/NEWS) for details.

This was [already done](https://tracker.debian.org/news/395661) in Debian.