Revert "cas: add use-application-default-credentials flag"
Revert "cas: add use-application-default-credentials flag"

This reverts commit 28351821a0b816c18f9d6b2500076b43e2166445.

Reason for revert:
I thought it is convinience if we can use ADC.
But I now don't think we need to have this after implementing realm support via vadim's comment.
https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2342166/3#message-09e527df11ae147841d9ccac18444ad9b72960de

Original change's description:
> cas: add use-application-default-credentials flag
> 
> Credetials from token server will be used by default.
> 
> Bug: 1110569
> Change-Id: I12e3b6f1079892182287d13332a1c5f909ad929f
> Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2342166
> Reviewed-by: Ye Kuang <yekuang@google.com>
> Commit-Queue: Ye Kuang <yekuang@google.com>
> Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
> Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

TBR=71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org,jwata@google.com,yekuang@google.com,infra-scoped@luci-project-accounts.iam.gserviceaccount.com

Change-Id: Ic99c1ae138ace75a0cd33b5d8b48c857221bc9eb
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1110569
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2342167
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go