Remove the known digest check from channel_check_for_duplicates()
This is in arma's commit f5ff9f23 in his bug24898-more branch.
This comment is wrong, the check is already implemented in channel_add_to_digest_map():
+ /* XXX we might want to replace this logic with a call to
+ * channel_is_client() instead. Or maybe we can change
+ * channel_identity_mapso it refuses to add any channels that are
+ * client channels? */
So we can remove the check entirely.