Update the basic graphics detail slider.

Looks like LL has had two ways of dealing with the basic graphics detail slider in preferences, which is typical of them.
And looks like they left crap laying around from the old method.

Removed the crap.
Updated the values the slider uses to better suit modern equipment, while leaving the low end the same.
Pumped up "linden" tree rendering, it was way too low.
Clean out no longer needed commented code.