Disable taskbar tabs

Follow up to #44267.

TL; DR:

• needs to turn off always-on PBM, currently not supported
• quite annoying UX on Tor Browser because of the default disabled remoting
• needs some polish on Mullvad Browser regarding uninstallation and other details.

So, we're just keeping this off for now.

The pref to set is browser.taskbarTabs.enabled.