swarming: separate out task reaping for external scheduler and simplify
swarming: separate out task reaping for external scheduler and simplify

The existing bot_reap_task flow includes a number of steps that are
either irrelevant or even incorrect when reaping from external
scheduler.

Separate out the reaping flow for external_scheduler reaping.

BUG=928944

Change-Id: Id5387d91e9d50b8e179df37b08d996857d451181
Reviewed-on: https://chromium-review.googlesource.com/c/1459525
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Commit-Queue: Aviv Keshet <178e8ae417efbe404feca7ab4cb8f0ba8c109369@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py