bpo-46752: Slight improvements to TaskGroup API (GH-31398)

* Remove task group names (for now)

We're not sure that they are needed, and once in the code
we would never be able to get rid of them.

Yury wrote:

> Ideally, there should be a way for someone to build a "trace"
> of taskgroups/task leading to the current running task.
> We could do that using contextvars, but I'm not sure we should
> do that in 3.11.

* Pass optional name on to task in create_task()

* Remove a bunch of unused stuff