Stop invoking secdiscard when deleting password data

secdiscard never works on recent devices; stop calling it to
reduce the false SELinux denials messages. Just logically
zeroize the data before unlinking it.

Bug: 62140539
Test: flash device; change PIN; observe no SELinux error messages.
Change-Id: I5f47fc81735a7d9995c2da9e52a25ae903db3ced