Revert "RPC: prevent crash with walletpassphrase"

This crash seemed to have never happened in 0.8.x, and we are missing the later commit
in a8db31c83d so it seems better to just revert this.

This reverts commit 036a69c13c.

src/bitcoinrpc.h

@@ -49,7 +49,6 @@ enum RPCErrorCode
     RPC_INVALID_PARAMETER           = -8,  // Invalid, missing or duplicate parameter
     RPC_DATABASE_ERROR              = -20, // Database error
     RPC_DESERIALIZATION_ERROR       = -22, // Error parsing or validating structure in raw format
-    RPC_SERVER_NOT_STARTED          = -18, // RPC server was not started (StartRPCThreads() not called)
 
     // P2P client errors
     RPC_CLIENT_NOT_CONNECTED        = -9,  // Bitcoin is not connected

src/rpcwallet.cpp

@@ -1363,8 +1363,6 @@ Value walletpassphrase(const Array& params, bool fHelp)
             "Stores the wallet decryption key in memory for <timeout> seconds.");
     if (fHelp)
         return true;
-    if (!fServer)
-        throw JSONRPCError(RPC_SERVER_NOT_STARTED, "Error: RPC server was not started, use server=1 to change this.");
     if (!pwalletMain->IsCrypted())
         throw JSONRPCError(RPC_WALLET_WRONG_ENC_STATE, "Error: running with an unencrypted wallet, but walletpassphrase was called.");