Fix SearchTask memleak

There's a autoDelete option for a task, but it's up to the task to implement how it will delete itself. For SearchHistoryTask this never happened.

I tested holding the search button for around one minute and the memory was slowly growing and never freed. Adding a destructor with a debug to remove any doubt made me realize: The search task was only removed when konsole closed.

This is not just about the Memory Management, as the search was also connected to multiple objects and could potentially mishandle searches.