Remove max turns from task

[jlowin]

It's very appealing to limit the number of turns for attempting a task by setting Task(max_turns=...) but unfortunately in practice this isn't straightforward. If a single agent is applied to the task, then it can iterate forever because there is no concept of formally "ending" a turn. On the other hand, we can't use LLM invocations because a turn usually requires two invocations - one to use a tool and another to process it. For the moment, enforcing operational limits at the Task level is too complex. We will leave run() limits in place.