qapi: Simplify QAPISchemaVariants @tag_member

qapi: Simplify QAPISchemaVariants @tag_member

For union types, the tag member is known only after .check().

We used to code this in a simple way: QAPISchemaVariants attribute
.tag_member was None for union types until .check().

Since this complicated typing, recent commit "qapi/schema: fix typing
for QAPISchemaVariants.tag_member" hid it behind a property.

The previous commit lets us treat .tag_member just like the other
attributes that become known only in .check(): declare, but don't
initialize it in .__init__().

Signed-off-by: Markus Armbruster <armbru@redhat.com>