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.
Merge pull request #622 from mitya57/master

Remove Metacity support from libsteticui