message: rpc: deduplicate WriteHDKeypath() used in decodepsbt
  The functionality is already provided in the BIP32 utility library util/bip32.h
  with the exact same name and function signature.
diff: #include <script/signingprovider.h>
  #include <script/standard.h>
  #include <uint256.h>
  +#include <util/bip32.h>
  #include <util/moneystr.h>
  #include <util/strencodings.h>
  #include <util/string.h>
  @@ -910,25 +911,6 @@ static UniValue testmempoolaccept(const JSONRPCRequest& request)
  return result;
  }
  -static std::string WriteHDKeypath(std::vector<uint32_t>& keypath)
  -{
  - std::string keypath_str = "m";
  - for (uint32_t num : keypath) {
  - keypath_str += "/";
  - bool hardened = false;
  - if (num & 0x80000000) {
  - hardened = true;
  - num &= ~0x80000000;
  - }
  -
  - keypath_str += ToString(num);
  - if (hardened) {
  - keypath_str += "'";
  - }
  - }
  - return keypath_str;
  -}
  -
  UniValue decodepsbt(const JSONRPCRequest& request)
  {
  RPCHelpMan{"decodepsbt",
  