wallet: Comment out HDSeed and CHDChain persistence to disk

Sapling keys generated from the seed are not yet persisted, so we don't
want to persist the seed or chain state either, otherwise the wallet
could end up in an inconsistent state.

Some tests are temporarily disabled because commenting out HDSeed
persistence breaks invariants inside CCryptoKeyStore.

Revert this commit during the PR for #3388.

src/test/rpc_wallet_tests.cpp

@@ -1257,6 +1257,7 @@ BOOST_AUTO_TEST_CASE(rpc_z_sendmany_internals)
 /*
  * This test covers storing encrypted zkeys in the wallet.
  */
+/* TODO: Uncomment during PR for #3388
 BOOST_AUTO_TEST_CASE(rpc_wallet_encrypted_wallet_zkeys)
 {
     LOCK2(cs_main, pwalletMain->cs_wallet);
@@ -1312,6 +1313,7 @@ BOOST_AUTO_TEST_CASE(rpc_wallet_encrypted_wallet_zkeys)
     // We can't simulate over RPC the wallet closing and being reloaded
     // but there are tests for this in gtest.
 }
+*/
 
 
 BOOST_AUTO_TEST_CASE(rpc_z_listunspent_parameters)