Hide screenshots menu since we don't support it

Tor Browser doesn't ship with with the system addon screenshots@mozilla.org.xpi for fingerprinting reasons - see #40007 (closed) - but the menu still exists and causes confusion - see https://forum.torproject.net/t/take-screenshot-right-click-and-select-appears-to-do-nothing/2834

We can solve this by setting extensions.screenshots.disabled = false true in 000-tor-browser.js.

@duncan

edited: false to true, my bad