upgrade_test: remove repeated fill_and_verify_db_data after upgrade
upgrade_test: remove repeated fill_and_verify_db_data after upgrade

Currently we run fill_and_verify_db_data twice after cluster is upgraded.

There is an verify test after upgrading last node in the upgrade for loop:
|       for i in indexes[1:]:
|           self.db_cluster.node_to_upgrade = self.db_cluster.nodes[i]
|           self.log.info('Upgrade Node %s begin', self.db_cluster.node_to_upgrade.name)
|           self.upgrade_node(self.db_cluster.node_to_upgrade)
|           self.log.info('Upgrade Node %s ended', self.db_cluster.node_to_upgrade.name)
|           self.db_cluster.node_to_upgrade.check_node_health()
| >>>>      self.fill_and_verify_db_data('after upgraded %s' % self.db_cluster.node_to_upgrade.name)

Signed-off-by: Amos Kong <eb01d8f828a6c9a20be4534e72e049aaf41503df@scylladb.com>

scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests