Remove 'destruct' from cipher_ops

We never delete a cipher, and the destruct op is empty, so it's a bit pointless

include/olm/cipher.h

@@ -92,9 +92,6 @@ struct _olm_cipher_ops {
         uint8_t const * ciphertext, size_t ciphertext_length,
         uint8_t * plaintext, size_t max_plaintext_length
     );
-
-    /** destroy any private data associated with this cipher */
-    void (*destruct)(struct _olm_cipher *cipher);
 };
 
 struct _olm_cipher {

src/cipher.cpp

@@ -131,17 +131,12 @@ size_t aes_sha_256_cipher_decrypt(
 }
 
 
-void aes_sha_256_cipher_destruct(struct _olm_cipher *cipher) {
-}
-
-
 const _olm_cipher_ops aes_sha_256_cipher_ops = {
   aes_sha_256_cipher_mac_length,
   aes_sha_256_cipher_encrypt_ciphertext_length,
   aes_sha_256_cipher_encrypt,
   aes_sha_256_cipher_decrypt_max_plaintext_length,
   aes_sha_256_cipher_decrypt,
-  aes_sha_256_cipher_destruct
 };
 
 } // namespace