Fully deprecate sys prop for aconfig flags

Aconfig new storage system has been enabled for several months. Turning
off syncing over to sys prop completely.

Test: m
Change-Id: Ic575d95f8a6d5d0c654d315f8fd707c055431b91
Merged-In: I51e9cf39b459e651c0bcc5b74445c660a44469f0