Revert "Ramp up cast_shell_linux on LUCI to 100%."

This reverts commit d553b24a.

Reason for revert: Too many pending builds, will reland once drained.

Original change's description:
> Ramp up cast_shell_linux on LUCI to 100%.
> 
> Bug: 731556
> Change-Id: Id416d395958f0b3529816f17ca0d35203f863065
> Reviewed-on: https://chromium-review.googlesource.com/950023
> Reviewed-by: John Budorick <jbudorick@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#541215}

TBR=jchinlee@chromium.org,jbudorick@chromium.org

Change-Id: I364807043e712551a28d9b2a6ebbe114eba1e0e7
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 731556
Reviewed-on: https://chromium-review.googlesource.com/952283Reviewed-by: Jao-ke Chin-Lee <jchinlee@chromium.org>
Commit-Queue: Jao-ke Chin-Lee <jchinlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541221}

infra/config/branch/cq.cfg

@@ -79,7 +79,7 @@ verifiers {
       name: "master.tryserver.chromium.linux"
       builders {
         name: "cast_shell_linux"
-        equivalent_to { bucket: "luci.chromium.try" percentage: 100 }
+        equivalent_to { bucket: "luci.chromium.try" percentage: 10 }
       }
       builders {
         name: "chromium_presubmit"