Remove frame rate flexibility token

Remove the frame rate flexiblity token which can be replaced with

    DisplayManager.setRefreshRateSwitchingType(
          DisplayManager.SWITCHING_TYPE_ACROSS_AND_WITHIN_GROUPS);
    DisplayManager.setShouldAlwaysRespectAppRequestedMode(true);

Bug: 175371491
Test: m

Test: manually test that backdoor still works
  adb shell dumpsys SurfaceFlinger | grep -A 15 "DesiredDisplayModeSpecs"
  verify no override

  adb shell service call SurfaceFlinger 1036 i32 1
  adb shell dumpsys SurfaceFlinger | grep -A 15 "DesiredDisplayModeSpecs"
  verify there is override

  adb shell service call SurfaceFlinger 1036 i32 0
  adb shell dumpsys SurfaceFlinger | grep -A 15 "DesiredDisplayModeSpecs"
  verify no override

  verify idempotence:
  adb shell service call SurfaceFlinger 1036 i32 0
  adb shell service call SurfaceFlinger 1036 i32 0
  adb shell service call SurfaceFlinger 1036 i32 1
  adb shell service call SurfaceFlinger 1036 i32 1

Change-Id: I848f849037da38d245e940bc9c85727129463a81