qapi: Delete useless check_exprs() code for simple union kind

qapi: Delete useless check_exprs() code for simple union kind

Commit bceae7697f "qapi script: support enum type as discriminator in
union" made check_exprs() add the implicit enum types of simple unions
to global @enum_types.  I'm not sure it was needed even then.  It's
certainly not needed now.  Delete it.

discriminator_find_enum_define() and add_name() parameter @implicit
are now dead.  Bury them.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190914153506.2151-17-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>