qapi: Drop obsolete tag value collision assertions

Union tag values can't clash with member names in generated C anymore
since commit e4ba22b, but QAPISchemaObjectTypeVariants.check() still
asserts they don't.  Drop it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1446559499-26984-1-git-send-email-armbru@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1447836791-369-5-git-send-email-eblake@redhat.com>
