Nuke the remainings of _notmuch_message_add_thread_id.

The function _notmuch_message_add_thread_id has been removed
from the private interface of notmuch. There's no reason for
one to keep a declaration of its prototype in the code base.
Also, lets update a commentary that referenced that function
and escaped from previous scrutiny.

Signed-off-by: Fernando Carrijo <fcarrijo@yahoo.com.br>