Removing SECURE_KEY_GPS_MODE flag.

It's not being used anywhere and it's probably not something we want
people to be using. The KEY_GPS_MODE flag still exists as a key in
Global.BATTERY_SAVER_CONSTANTS.

Test: atest com.android.server.power.BatterySaverPolicyTest
Change-Id: If437b427b4574c15e4cb75d85a25f7aff514c102