SurfaceFlinger: remove mUse90Hz flag

As we have now DisplayManager changes in place (b/122905403)
there is no need for mUse90Hz flag. 90Hz will be enabled by
DisplayManager.

Fixes: 126725468
Test: Enable peak refresh from settings
Change-Id: I949727f8ba41bba874ab07160f3f6153d3bf4c67
4 files changed