Clean up QmlEngine specific code from debbuger plugin.
The logic to check if the current debug engine is qmlengine
is done by scriptconsole. It also listens to debug engine
state transitions to enable/disable itself accordingly.

Change-Id: If21b1167e870e87166b2f7fd6f3283063e47ded0
Reviewed-by: Kai Koehne <kai.koehne@nokia.com>