refactor: Move GetDifficulty out of rpc/server.h

It has no business in `rpcserver.h`. Define it in the interface header
of the implementation unit `rpcblockchain` where it is defined.

Also modernize the signature to:

    double GetDifficulty(const CBlockIndex* blockindex = nullptr);

(remove `extern`, replace `NULL` with `nullptr`)

src/rpc/blockchain.cpp

@@ -3,6 +3,8 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
+#include "rpc/blockchain.h"
+
 #include "amount.h"
 #include "chain.h"
 #include "chainparams.h"
@@ -42,13 +44,6 @@ static CUpdatedBlock latestblock;
 extern void TxToJSON(const CTransaction& tx, const uint256 hashBlock, UniValue& entry);
 void ScriptPubKeyToJSON(const CScript& scriptPubKey, UniValue& out, bool fIncludeHex);
 
-/**
- * Get the difficulty of the net wrt to the given block index, or the chain tip if
- * not provided.
- *
- * @return A floating point number that is a multiple of the main net minimum
- * difficulty (4295032833 hashes).
- */
 double GetDifficulty(const CBlockIndex* blockindex)
 {
     if (blockindex == NULL)