remove is_buddylist from the irssi window manager

remove the is_buddylist function and usage from the irssi window manager. Fixes LIBGNT-6

Testing Done:
Ran most of the demos, namely multiwin with the irssi window manager to make sure everything was still working.

Bugs closed: LIBGNT-6

Reviewed at https://reviews.imfreedom.org/r/199/