qapi: Drop one of two "simple union must not have base" checks

The first check ensures the second one can't trigger.  Drop the first
one, because the second one is in a more logical place, and emits a
nicer error message.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
