klipper: remove the "disable this popup" menu item

Reason for the change

My sense is that this was added during the time when there were clipboard actions included by default, because we wanted to provide a way to disable this potentially unexpected and intrusive popup.

However we haven't shipped clipboard actions for a long time, at least 8 years. Thus, almost everyone using them has set them up manually. This means if they did want to disable the popup, they already know how from the config window.

Let's remove it.

Test Plan

1. Enable "Show action popup menu [X] Immediately on selection" in Klipper's settings window
2. Copy a URL

Bugs fixed

BUG: 412264
BUG: 503862
FIXED-IN: 6.4.0