[BACKPORT][v1.5.4][IMPROVEMENT] Clean up backup target in IM-R pod if the backup target setting is unset

[github-actions]

backport #5741

[longhorn-io-github-bot]

Pre Ready-For-Testing Checklist

• Where is the reproduce steps/test steps documented?
  The reproduce steps/test steps are at:

  • steps:
    • Set BackupTarget
    • Create Volume with one replica(so you know which instance-manage) and Create Backup
    • Exec into instance-manager: mount | grep "nfs", you will see the mount points
    • Unset the BackupTarget
    • Exec into instance-manager: mount | grep "nfs", the mount points is unmount

• Have the backend code been merged (Manager, Engine, Instance Manager, BackupStore etc) (including backport-needed/*)?
  The PR is at

  • longhorn-instance-manager: feat(backuptarget): add cleanup all mounts grpc function (backport #305) longhorn-instance-manager#324
  • longhorn-manager: feat(backuptarget): add cleanup all mounts grpc function (backport #2279) longhorn-manager#2320

[chriscchien]

Verified pass on longhorn v1.5.x (longhorn-manager ab4a0f, longhorn-instance-manager 1f0737) with test steps

Can find nfs mount in instance-manager pod when backup target is nfs and have backup created. After unset the backup target, the nfs mount in instance-manager pod gone.