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
Merge "Removes BuiltInstanceTasks.get_volume_mountpoint"