Make extensions.torbutton.launch_warning be taken into account (refs: #15707)

For some reason, it's ignored when we set it via
/usr/share/tails/tor-browser-prefs.js.

config/chroot_local-includes/etc/tor-browser/profile/prefs.js

+// Prefs that *need* to be here because they are not honored
+// if we set them via /usr/share/tails/tor-browser-prefs.js
+user_pref("extensions.torbutton.launch_warning",  false);

config/chroot_local-includes/usr/share/tails/tor-browser-prefs.js

 // As suggested in TBB's start-tor-browser script for system-wide Tor
 // instances
 pref("network.security.ports.banned", "631,6136,4444,4445,6668,7656,7657,7658,7659,7660,8998,9040,9050,9062,9150,9051");
-pref("extensions.torbutton.launch_warning",  false);
 
 // Tails-specific configuration below