I have been looking for a solution to this for a while, as it appears many have. But I believe I have worked it out.
Create a new ui policy on the table or tables required, with no condition, and add a ui policy action to make work_notes field mandatory - example:

When you check back on the workspace you will see that both buttons for comment and work notes have gone:

Of course, it means that work notes always need something added when performing an update, but that should be best practice anyway.

Be aware!  if you have followed this Servicenow article Disable the Post Comments button from disappearing in Activity stream (servicenow.com) and set the value field in  glide.activity.compose.can_post_mandatory_fields system property to true - the above ui policy will have no effect.