Remove ximagesink and xvimagesink from Pidgin options as they're broken

At the time of this commit, ximagesink and xvimagesink are broken
when embedding into a Gtk window. They display as a black screen
or artifacts of not being drawn.

This commit doesn't disallow them completely as the default option
still uses autovideosink, which can choose either of these sinks
still. However, it's probably a good idea to remove known broken
sinks to from the plugin options list.