remove no-op notifyFailureDetector from GDSVH

See discussion at http://mail-archives.apache.org/mod_mbox/cassandra-dev/201301.mbox/%3CCABT57mYXOqSU2Md4xKiv9c5P7d5-PuBJFUkQtKvKY0EkuvCmbw@mail.gmail.com%3E