qapi: Drop obsolete tag value collision assertions

Union tag values can't clash with member names in generated C anymore
since commit e4ba22b3, 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>

scripts/qapi.py

@@ -1057,8 +1057,7 @@ def check(self, schema, members, seen):
             assert self.tag_member in seen.itervalues()
         assert isinstance(self.tag_member.type, QAPISchemaEnumType)
         for v in self.variants:
-            vseen = dict(seen)
-            v.check(schema, self.tag_member.type, vseen)
+            v.check(schema, self.tag_member.type, {})
 
 
 class QAPISchemaObjectTypeVariant(QAPISchemaObjectTypeMember):