RPC: getblocktemplate does not require a key, to create a block template

getblocktemplate only uses certain portions of the coinbase transaction,
notably ignoring the coinbase TX output entirely.

Use CreateNewBlock() rather than CreateNewBlockWithKey(), eliminating
the needless key passing.

Should be zero behavior changes.

src/rpcmining.cpp

@@ -280,7 +280,8 @@ Value getblocktemplate(const Array& params, bool fHelp)
             delete pblocktemplate;
             pblocktemplate = NULL;
         }
-        pblocktemplate = CreateNewBlockWithKey(*pMiningKey);
+        CScript scriptDummy = CScript() << OP_TRUE;
+        pblocktemplate = CreateNewBlock(scriptDummy);
         if (!pblocktemplate)
             throw JSONRPCError(RPC_OUT_OF_MEMORY, "Out of memory");