[validation] Remove unused first_invalid parameter from ProcessNewBlockHeaders()

No callers use the returned value in first_invalid. Remove it from the
function signature and don't set it in the function.

src/rpc/mining.cpp

@@ -773,7 +773,7 @@ static UniValue submitheader(const JSONRPCRequest& request)
     }
 
     BlockValidationState state;
-    ProcessNewBlockHeaders({h}, state, Params(), /* ppindex */ nullptr, /* first_invalid */ nullptr);
+    ProcessNewBlockHeaders({h}, state, Params());
     if (state.IsValid()) return NullUniValue;
     if (state.IsError()) {
         throw JSONRPCError(RPC_VERIFY_ERROR, FormatStateMessage(state));