KeyStore: Remove duplicate method

Remove the duplicate() method from KeyStore.
It is backed by dead code in the Keystore service, which (as far as I
can tell) is not doing the right thing.

Previous conversations with Keystore team members suggested this API
should not be used and it is marked for removal in the Keystore service.

Bug: 72037261
Test: That it compiles.
Change-Id: I7f8af95473c876340cbd5c73dd88c5d0282897b3