Should hide message display action button when the currently displayed message isn't scheduled

[jikamens]

At some point the "Send Later" button started showing up in the message header buttons in the preview pane. It wasn't there before and nothing has changed in the add-on, so this appears to have been caused by a change in Thunderbird. It shouldn't be there; it serves no purpose and isn't even active (i.e., it can't be clicked). I need to figure out why it's there and how to make it go away.

[jikamens]

Oh, I was wrong, this is a different button that is clickable for scheduled messages, and when you click it it pops up a window telling you the scheduled send time of the message. I need to figure out how to make the button go away automatically when the current message isn't a scheduled message.

[jikamens]

Blocked by upstream: https://bugzilla.mozilla.org/show_bug.cgi?id=1940274