remove 0421: privacy.trackingprotection.ui.enabled
remove 0421: privacy.trackingprotection.ui.enabled

- pref removed in FF63 (https://bugzilla.mozilla.org/1476879)
- when we added it the default was false
- default is true since FF57
- it's only an UI thing

ergo we don't need to move it to 9999
ghacksuserjs/ghacks-user.js,ghacksuserjs/ghacks-user.js