Remove bp:ipv6-subnet-attributes (superseded)

Removing the file ipv6-subnet-attributes.rst as the work would now be
implemented as part of bp:ipv6-api-testing-parity as discussed in the
patch https://review.openstack.org/#/c/102468/

Change-Id: Ib4330d7dc29cdbd0cea466534aff0afeb72108a6