plugins: Drop support for old style python plugins

All the the old python plugins have already been removed. Remove a bunch of the
other related cruft since we want to port the plugin system to libpeas

Taken from 65371f53db (diff-2bbd410468516599f21e84fecb500348)

tools/plugin_template/xed-plugin.desktop.in

@@ -1,7 +1,4 @@
 [Xed Plugin]
-##ifdef WITH_PYTHON
-Loader=python
-##endif
 Module=##(PLUGIN_MODULE)
 IAge=2
 _Name=##(PLUGIN_NAME)