pidgin/purple-plugin-pack

Remove plugin_pack.spec.in as we haven't provided rpms in a long time

Testing Done:
Ran `ninja dist` and verified no issues.

Reviewed at https://reviews.imfreedom.org/r/2404/
if TYPES.contains('default') and PIDGIN.found()
difftopic = shared_module('difftopic',
'difftopic.c',
dependencies : [PIDGIN, GTK],
name_prefix : '',
install : true,
install_dir : PIDGIN_LIBDIR)
PP_PIDGIN_BUILD += 'difftopic'
endif