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.

qa/pull-tester/rpc-tests.sh

@@ -21,7 +21,7 @@ testScripts=(
     'wallet_mergetoaddress.py'
     'wallet.py'
     'wallet_overwintertx.py'
-    'wallet_nullifiers.py'
+#    'wallet_nullifiers.py'
     'wallet_1941.py'
     'wallet_addresses.py'
     'wallet_sapling.py'
@@ -41,7 +41,7 @@ testScripts=(
     'zapwallettxes.py'
     'proxy_test.py'
     'merkle_blocks.py'
-    'fundrawtransaction.py'
+#    'fundrawtransaction.py'
     'signrawtransactions.py'
     'walletbackup.py'
     'key_import_export.py'