I must be doing this wrong. When I visit the "move" form, and add "Removed Posts" to the "Destination" field, it supplies a selection of the word "removed." If I select that and click the blue "Move Post" button at the bottom of the page nothing happens. In this scenario, the radio button is selected for "Create a new topic, article, idea, or entry." (This doesn't seem like the correct selection.)
Next, I tried changing the radio button to "Merge with an existing topic, article, idea, or entry." That changes the "Destination" field to a field labeled "Destination Post ID." Since the phrase "Removed Posts" isn't an integer, I didn't expect that to work, but I tried it anyway. Again, clicking the blue "Move Post" button at the bottom of the page doesn't result in any error message or action.
Here's the comment I'm trying to move/delete if anyone else wants to try it. https://community.atlassian.com/t5/JIRA-Service-Desk-questions/Within-Activity-How-do-I-comment-a-participants/qaq-p/617105#M5789 (Comment from "web.com JIRA Admin")
I'm still seeing some posts where "delete my own" would be good to have. A recent example was someone posting a "Thanks, that worked!" comment to an answer. They accidentally posted it as an answer instead. Then they did post that as a comment to my answer, but were unable to delete their accidental-answer and just edited it to say "<duplicate message>".