[swarming] cleanup backcompat processing of task to run with try_number
[swarming] cleanup backcompat processing of task to run with try_number

Change-Id: I7156cef6492249b6c438d94df2a46afceda17aed
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/3946641
Auto-Submit: Jonah Hooper <2346c1b958e225fb7eb5e3a6fe644e71150a8db1@google.com>
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

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