Fix bug with deleting private messages.
Fix bug with deleting private messages.

This was due to an AttributeError occurring when getting the guild_id
from the channel. PrivateChannels do not have guild_ids so they should
be None.

Rapptz/discord.py,Harmon758/discord.py,rapptz/discord.py,mgardne8/discord.py,lgkern/PriestPy,variablehair/Eggplantato,khazhyk/discord.py,Harmon758/discord.py,imayhaveborkedit/discord.py