Now that navigator.platform has been frozen on the Android and Linux/etc, SPOOFED_PLATFORM can be removed because the old spoofed values match the new frozen values for all platforms: Windows, macOS, Android, and Linux/etc.
Even though navigator.platform's default and spoofed values are always the same, this change keeps RFPTarget::NavigatorPlatform enum value so we know whether to honor the "general.platform.override" pref: the pref should override the default platform, but the spoofed platform should override the pref.
Depends on D220360
Differential Revision: https://phabricator.services.mozilla.com/D220361
ba502a3440