Remove g_rpc_chain global

Replace with RPC request reference to new WalletContext struct similar to the
existing NodeContext struct and reference.

This PR is a followup to 25ad2c623a
https://github.com/bitcoin/bitcoin/pull/18740 removing the g_rpc_node global.

Some later PRs will follow this up and move more wallet globals to the
WalletContext struct.

Co-authored-by: João Barbosa <joao.paulo.barbosa@gmail.com>

src/wallet/context.cpp

@@ -0,0 +1,8 @@
+// Copyright (c) 2020 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <wallet/context.h>
+
+WalletContext::WalletContext() {}
+WalletContext::~WalletContext() {}