qapi: Stop rejecting #optional

Commit 1d8bda1 got rid of #optional tags, and added a check to keep
them from getting added back, to make sure patches then in flight
don't add them back.  It's been six months, time to drop that check.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20171002141341.24616-3-armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
