qapi: Clean up after removal of simple unions

Commit 4e99f4b12c0 (qapi: Drop simple unions) missed a bit of code
dealing with simple union branches.  Drop it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20230316071325.492471-4-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
1 file changed