Remove unused group label destroy HAML js

What does this MR do?

closes #250845 (closed)

In the interest of removing inline js - this MR removes app/views/groups/labels/destroy.js.haml and cleans up the controller.

The label removal is tested in https://gitlab.com/gitlab-org/gitlab/blob/master/spec/features/projects/labels/user_removes_labels_spec.rb#L5

The pipeline passes and I couldn't find anywhere else this action might be hit with a javascript request, so I think this is completely unused.

Conformity

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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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