wallet: Disallow creating legacy wallets

Remove the option to set descriptors=False when creating a wallet, and
enforce this in RPC and in CreateWallet

test/functional/wallet_listdescriptors.py

@@ -33,11 +33,6 @@ class ListDescriptorsTest(BitcoinTestFramework):
         node = self.nodes[0]
         assert_raises_rpc_error(-18, 'No wallet is loaded.', node.listdescriptors)
 
-        if self.is_bdb_compiled():
-            self.log.info('Test that the command is not available for legacy wallets.')
-            node.createwallet(wallet_name='w1', descriptors=False)
-            assert_raises_rpc_error(-4, 'listdescriptors is not available for non-descriptor wallets', node.listdescriptors)
-
         self.log.info('Test the command for empty descriptors wallet.')
         node.createwallet(wallet_name='w2', blank=True, descriptors=True)
         assert_equal(0, len(node.get_wallet_rpc('w2').listdescriptors()['descriptors']))