Removes BuiltInstanceTasks.get_volume_mountpoint
Removes BuiltInstanceTasks.get_volume_mountpoint

Reasons:
- Method is not being used in the project.
- Method has an unassigned variable used volume_id.

Changes:
- Removes get_volume_mountpoint from trove.taskmanager.models.

Change-Id: I2942a4cc952a473dd1ed1d52c8cb2205566b68ae
Closes-Bug: #1263371

mmasaki/trove,openstack/trove,fabian4/trove,redhat-openstack/trove,redhat-openstack/trove,zhangg/trove,cp16net/trove,zhujzhuo/openstack-trove,denismakogon/trove-guestagent,zhujzhuo/openstack-trove,denismakogon/trove-guestagent,mmasaki/trove,changsimon/trove,redhat-openstack/trove,cp16net/trove,cp16net/trove,zhangg/trove,mmasaki/trove,zhujzhuo/openstack-trove,hplustree/trove,fabian4/trove,changsimon/trove,changsimon/trove,hplustree/trove,openstack/trove,fabian4/trove