message: Fix a clang warning in wallet serialization method
  The warning was due to a self assignment and this is
  how the warning messages looked like:
  In file included from ../../src/wallet/test/walletdb_tests.cpp:9:
  In file included from ../../src/wallet/wallet.h:21:
  ../../src/wallet/walletdb.h:64:18: warning: assigning field to itself [-Wself-assign-field]
  nVersion = this->nVersion;
  ^
diff: @@ -61,7 +61,6 @@ public:
  inline void SerializationOp(Stream &s, Operation ser_action)
  {
  READWRITE(this->nVersion);
  - nVersion = this->nVersion;
  READWRITE(nExternalChainCounter);
  READWRITE(masterKeyID);
  }
  