Remove (broken) "unlinking segmentation layer" feature

[daniel-wer]

Todo

Remove the unlinking feature. See the following comment.

Context

I encountered this error on a dev instance and using the current master while trying to unlink a dataset's original segmentation layer in a hybrid annotation.

Expected Behavior

The unlinking should succeed.

Current Behavior

The page ends up in an error state with a red toolbar, showing a toast "Failed to save annotation. Retrying."
Sometimes, another toast with an error message is shown:

Failed to check data access, token may be expired, consider reloading.
Show more information
[Server Time 2022-04-06 11:49]  <~ Unsuccessful WS request to http://localhost:9000/api/tracingstores/localhost/validateUserAccess (ID: 112).Status: 400. Response: {"messages":[{"error":"Annotation couldn￢ﾀﾙt be found"}]}

Reloading the page shows the annotation with unlinked segmentation layer, so it looks like the action actually succeeded.

Steps to Reproduce the bug

1. Create a hybrid annotation on a dataset with a segmentation layer.
2. Select "Unlink a dataset's original segmentation layer" in the layer settings of the segmentation layer

Your Environment for bug

• Version of WebKnossos (Release or Commit): 22.04.0

[philippotto]

I think, the main (only?) use case for this feature was that clicking "create annotation" always created a volume layer with fallback and removing that fallback should be possible somehow. In the meantime, the user can simply click "add volume layer" to get a fresh one. Therefore, I think, we can remove the unlinking feature completely. I renamed the issue therefore. Feel free to veto!