Remove empty startup message from console

Change >> to >>> as in a normal Python console.
Avoid printing an empty string which is why the console looked like:
>>
Network banner
>>

Instead of:
Network banner
>>>

electrum/gui/qt/console.py

@@ -44,7 +44,7 @@ class OverlayLabel(QtWidgets.QLabel):
 
 
 class Console(QtWidgets.QPlainTextEdit):
-    def __init__(self, prompt='>> ', startup_message='', parent=None):
+    def __init__(self, prompt='>>> ', parent=None):
         QtWidgets.QPlainTextEdit.__init__(self, parent)
 
         self.prompt = prompt
@@ -56,7 +56,6 @@ class Console(QtWidgets.QPlainTextEdit):
         self.setWordWrapMode(QtGui.QTextOption.WrapAnywhere)
         self.setUndoRedoEnabled(False)
         self.document().setDefaultFont(QtGui.QFont(MONOSPACE_FONT, 10, QtGui.QFont.Normal))
-        self.showMessage(startup_message)
 
         self.updateNamespace({'run':self.run_script})
         self.set_json(False)