Rename secret_password_remove() to secret_password_clear()

Rename secret_password_remove() to secret_password_clear()

 * It's clearer what happens here: we try to remove as many matching
   passwords as possible.
 * Also rename secret_service_remove() to secret_service_clear()
 * Rename secret_password_clear() which used to wipe password memory
   to secret_password_wipe().