Remove pidgin_dialogs_im_with_user

I ported the the two remaining users of it to use purple_contact_find_dm and
purple_contact_create_dm_async.

Testing Done:
Ran the turtles and tested the notification from the demo protocol plugin.

This is a bit bugged out right now in the demo protocol plugin, as the new code requires that a PurpleContact exists, but the demo protocol plugin isn't creating them for the add user notifications.

Reviewed at https://reviews.imfreedom.org/r/3108/