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.