[isolate] Use go isolate binary

The isolate.py script is deprecated and should no longer be used. The
new go binary has feature parity and is distributed via CIPD instead of
via git.

* Remove unused parent_isolate feature.
* Remove custom gclient hack to checkout swarming_client.
* Remove build/swarming_client dependency.

Bug: https://crbug.com/1082177
Change-Id: I9b4870300e1b2269f9b338b89546cafaa393a5ce
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/148520
Reviewed-by: Karl Klose <karlklose@google.com>
18 files changed