Revert "fail if rbe_exec_root is set with use_siso=true"

This reverts commit 9436c1142c5404ffd24be354c289c0a9064abb76.

Reason for revert: break ChromeOS builders b/402483181

Original change's description:
> fail if rbe_exec_root is set with use_siso=true
>
> Siso doesn't handle rbe_exec_root.
>
> Bug: 402060808, 313567009
> Change-Id: Idf97c984f57d51414fa5233c0ce182182f2f5c8b
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6342492
> Auto-Submit: Fumitoshi Ukai <ukai@google.com>
> Commit-Queue: Junji Watanabe <jwata@google.com>
> Reviewed-by: Junji Watanabe <jwata@google.com>
> Commit-Queue: Fumitoshi Ukai <ukai@google.com>
> Cr-Commit-Position: refs/heads/main@{#1430690}

Bug: 402060808, 313567009, 402483181
Change-Id: I815009b3d48ced9c63a78df3376620524684bbd4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6343401
Reviewed-by: Fumitoshi Ukai <ukai@google.com>
Reviewed-by: Junji Watanabe <jwata@google.com>
Commit-Queue: Jeffrey Young <cowmoo@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Philipp Wollermann <philwo@google.com>
Cr-Commit-Position: refs/heads/main@{#1431248}