Remove toggle for display brightness apis.

Sysfs and drm paths for setting brightness all occur as part of layer
commands, and the separate brightness api is being remove from the
composer definition, so remove support in the framework.

Bug: 210151839
Test: builds, boots
Test: Modulate display brightness slider
Change-Id: I115b9cfa373ccbd4c541c67a08f0fec13c0c95ab