Revert "Add [unknown] irc user for handling messages from unknown sources"

This reverts commit 53cf811f6f17fec431d1a0112f498b7ff74fa12d.

Conflicts:
	irc_im.c
bee-chat: create temporary users for unknown chat participants

The imcb_chat_msg() function is unable to send messages to a chat with
a user who was not previously added. This function should allow for the
sending of messages with users who are not added. This is suitable for
protocols which are sending messages to a chat from random users or a
large amount of users which join and part frequently.

Conflicts:
	protocols/bee_chat.c