output_type_enum() fails when *obj is not a valid value of the enum
type.  Should not happen.  Drop the check, along with its unit tests.
This unmasks qapi_enum_lookup()'s assertion.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200424084338.26803-9-armbru@redhat.com>
[Commit message tweaked]