[vm/service] Remove Stack.awaiterFrames

This field was supposed[1] to replace Stack.asyncCausalFrames but IDEs
and other service protocol users never adopted it.

The mechanism for collecting awaiterFrames and asyncCausalFrames
was originally considerably different, but we have since unified
both[2] after we switched to lazy async stack traces[3].

Today the only differences between the two are:

- asyncCausalFrames represens async gaps explicitly, while
  awaiterFrames does not;
- when asynchronous function is running its synchronous part
  before the first suspension awaiterFrames will still represent
  the corresponding frame as an asynchronous activation, while
  asyncCausalFrames will treat the same frame as regular frame.

Consequently having this field does not add any value.

This CL removes it from the response and updates all tests to
test against asyncCausalFrames.

[1]: https://codereview.chromium.org/2692803006/
[2]: https://dart-review.googlesource.com/c/sdk/+/167561
[3]: https://github.com/dart-lang/sdk/issues/37668

Tested: pkg/vm_service, service, service_2
Bug: https://github.com/dart-lang/sdk/issues/51763
Change-Id: If2b035a8840047423b8ed8ce3b5d81155e9f38d0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/301062
Commit-Queue: Slava Egorov <vegorov@google.com>
Reviewed-by: Ben Konyi <bkonyi@google.com>