qapi: Remove unwanted commas after #optional keyword

We don't want that commas to be part of the generated documentation,
so remove them.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Message-Id: <20170122145407.27476-1-sw@weilnetz.de>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>