Remove backward compatibility for old ActionsMixin `actions` attribute

[jeremystretch]

Proposed Changes

The old actions and action_perms attributes on the ActionsMixin class were merged in v3.7 under #13550, with backward compatibility retained for the v3.7 release. This change removes that backward compatibility.

Justification

Removes obsolete code.

Impact

Any plugins which utilize ActionsMixin will need to update their relevant view classes to use the new declaration format for actions. See #13550 for examples.