clear history when using clear button in RPC console

- current code does not clear history, when using the clear button, this
  is added
- remove a currently unused variable but add a TODO comment

src/qt/rpcconsole.cpp

@@ -15,12 +15,11 @@
 
 #include <openssl/crypto.h>
 
+// TODO: add a scrollback limit, as there is currently none
 // TODO: make it possible to filter out categories (esp debug messages when implemented)
 // TODO: receive errors and debug messages through ClientModel
 
-const int CONSOLE_SCROLLBACK = 50;
 const int CONSOLE_HISTORY = 50;
-
 const QSize ICON_SIZE(24, 24);
 
 const struct {
@@ -269,6 +268,8 @@ static QString categoryClass(int category)
 void RPCConsole::clear()
 {
     ui->messagesWidget->clear();
+    history.clear();
+    historyPtr = 0;
     ui->lineEdit->clear();
     ui->lineEdit->setFocus();