Remove entire PurpleDebugUiInterface and implementations
The `purple_debug*` functions are routed through GLib's logging interface, and the UIs capture logs from there, so there's no need for this interface any more.
Testing Done:
Opened Debug Window and saw logs appearing there.
Reviewed at https://reviews.imfreedom.org/r/1074/
# UPDATING VERSION NUMBERS FOR RELEASES
#
# The version number is:
# <major>.<minor>.<micro><extra>
#
# micro += 1
#
# If any functions have been added to libpurple, Pidgin, or Finch:
# micro = 0
# minor += 1
#
# If backwards compatibility has been broken in libpurple, Pidgin, or Finch:
# micro = 0
# minor = 0
# major += 1
# purple_soversion += 1
#
# extra should be similar to one of the following: