swarming: remove _EXECUTORS_PER_SHARD
swarming: remove _EXECUTORS_PER_SHARD

I think we don't need to have this item number limit.

Bug: 1151408
Change-Id: I7372bd0769ca9aa71c0251da12911e6c57a6a3a8
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2695249
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

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