remove unnecessary flush from upgrade tests
remove unnecessary flush from upgrade tests

drain triggers flush, so this isn't necessary
though at a later time we might want to test what
happens when nodes aren't drained and upgrades are attempted

krummas/cassandra-dtest,aweisberg/cassandra-dtest,riptano/cassandra-dtest,snazy/cassandra-dtest,mambocab/cassandra-dtest,blerer/cassandra-dtest,pcmanus/cassandra-dtest,bdeggleston/cassandra-dtest,spodkowinski/cassandra-dtest,pauloricardomg/cassandra-dtest,krummas/cassandra-dtest,stef1927/cassandra-dtest,carlyeks/cassandra-dtest,tjake/cassandra-dtest,carlyeks/cassandra-dtest,aweisberg/cassandra-dtest,thobbs/cassandra-dtest,thobbs/cassandra-dtest,riptano/cassandra-dtest,beobal/cassandra-dtest,mambocab/cassandra-dtest,iamaleksey/cassandra-dtest,bdeggleston/cassandra-dtest,yukim/cassandra-dtest,stef1927/cassandra-dtest,iamaleksey/cassandra-dtest,snazy/cassandra-dtest,pauloricardomg/cassandra-dtest,spodkowinski/cassandra-dtest,blerer/cassandra-dtest,beobal/cassandra-dtest