Delete backup encryption code

Previously there was a plan to move the backup encryption logic to the
framework. However this plan was abandoned and encryption logic lives in
GMS now. This code is unused and out of date.

I'm removing this now because it blocks ag/19967045.

Test: m
  atest BackupFrameworksServicesRoboTests
  atest CtsBackupHostTestCases
Bug: 210083465

Change-Id: Ib5dc37898fde5c8ef9ea792931404a1915eda8aa