ext4_utils: define keyctl_unlink()

This will allow vold to unlink keys rather than revoking them.

This also removes the keyctl_revoke() definition for now, to discourage
any further (mis)use in the context of ext4 encryption.

Bug: 35988361
Change-Id: Ic94329040f77ffbe2415287fe1202fa61e8a006b
(cherry picked from commit 3f062383deb3e5ba7b16d4728f5dbc44f40bc178)
2 files changed