Performance settings: remove dithering setting

This setting has no effect in CM10.2, as SurfaceFlinger has now been
taught to automatically Do The Right Thing [tm] with regards to deciding
when to enable surface dithering.

Change-Id: I67fba732440507c1209f2793d826badd4b4d34d5
Merge remote-tracking branch 'cm/cm-10.2' into cm-10.2