Stop using the "stretching" file

As a small optimization and code simplification, stop reading and
writing the "stretching" file alongside each stored key.  vold never
does key stretching anymore.

There was one special case in the code where if the stretching file
existed and contained "nopassword", then the secret was ignored.
However, this didn't seem to be of any use, especially since it didn't
cause Keystore to be used, so it did *not* allow a key stored with no
secret to be read if a secret was unexpectedly provided.

Bug: 232452368
Bug: 251131631
Bug: 251147505
Change-Id: I5a7cbba7492526e51c451f222b9413d9fae6bce5