Don't use a secdiscardable file for keys encrypted by SP

Storage keys that are encrypted by the user's synthetic password don't
need to be securely deletable by vold, since secure deletion is already
implemented at a higher level: the synthetic password protectors managed
by LockSettingsService.  Therefore, remove the use of the secdiscardable
file by vold in this case to improve performance.

Bug: 232452368
Bug: 251131631
Bug: 251147505
Change-Id: I847d6cd3b289dbeb1ca2760d6e261a78c179cad0