Remove the Pidgin icon themes which are covered by XDG icon themes now and remove the stock api as that's all that was using it yet.

Testing Done:
* Built everything, including the docs and pidgin-pot
* Verified that the preferences window still functions properly
* Verified that the user list in the conversation window had properly sized icons
* Verified all of the status icons were not affected.

Reviewed at https://reviews.imfreedom.org/r/1088/