user-util: remove unsed NOLEGACY define

AFAICT, it was never hooked up to meson, so it'd only work if somebody manually
defined the flag. I think it's preferable to drop the ifdef: it removes only a
single access call to a fixed location, which is realy cheap. If poeple don't
want to make use of this, they should just not create the file. Making this
both compile-time optional and opt-in in the filesystem is unnecessary.