[TASK] Remove v2 volume offline rebuilding in Longhorn v1.7

[derekbit]

What's the task? Please describe

After discussing with @shuo-wu , there are some changes in the rebuilding related SPDK APIs. To make the integration of delta rebuilding simpler, we will remove v2 volume offline rebuilding in v1.7.0.

Offline rebuilding is still useful for some scenarios. We can add it back after settling down the design and the implementation of delta rebuilding.

Describe the sub-tasks

Additional context

[derekbit]

cc @shuo-wu @DamiaSan

[innobead]

Let's have another issue for adding offline rebuilding back and add it to 1.8.0.

[longhorn-io-github-bot]

Pre Ready-For-Testing Checklist

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

Shouldn't impact the regression test.

• Does the PR include the explanation for the fix or the feature?

• Does the PR include deployment change (YAML/Chart)? If so, where are the PRs for both YAML file and Chart?
  The PR for the YAML change is at:
  The PR for the chart change is at:

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

longhorn/longhorn-manager#2743

• Which areas/issues this PR might have potential impacts on?
  Area: v2 volume, offline rebuild
  Issues

[DamiaSan]

To make the integration of delta rebuilding simpler, we will remove v2 volume offline rebuilding in v1.7.0.

Do you mean online rebuilding instead delta rebuilding?