qapi: The #optional tag is redundant, drop

We traditionally mark optional members #optional in the doc comment.
Before commit 3313b61, this was entirely manual.

Commit 3313b61 added some automation because its qapi2texi.py relied
on #optional to determine whether a member is optional.  This is no
longer the case since the previous commit: the only thing qapi2texi.py
still does with #optional is stripping it out.  We still reject bogus
qapi-schema.json and six places for qga/qapi-schema.json.

Thus, you can't actually rely on #optional to see whether something is
optional.  Yet we still make people add it manually.  That's just
busy-work.

Drop the code to check, fix up and strip out #optional, along with all
instances of #optional.  To keep it out, add code to reject it, to be
dropped again once the dust settles.

No change to generated documentation.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1489582656-31133-18-git-send-email-armbru@redhat.com>
