Remove unused constants and cleanup KeyStorage.cpp

Now that the salt and hardware auth token related code has been removed,
we can remove the associated (and now unused) constants.

Also cleanup some comments and remove includes related to hardware auth
token support.

Bug: 181910578
Test: Cuttlefish boots.
Change-Id: I3733d5c6bbf6989adc165c554ee53faa2484f4b6
2 files changed