Revert "Reenable three iOS CQ builders."

This reverts commit da27a978.

Reason for revert: Still over capacity. Will investigate further next week.

Original change's description:
> Reenable three iOS CQ builders.
> 
> Overall cycle time and compile step time have improved signficantly
> on all three matching waterfall bots since the hardlink workaround
> landed yesterday:
>   - ios-device: http://shortn/_ow7Hr0gN6e
>   - ios-device-xcode-clang: http://shortn/_qdlv49pNuS
>   - ios-simulator-xcode-clang: http://shortn/_LlpTYE0H51
> 
> Bug: 740301, 739556
> Change-Id: Iebf1b1398273d87580d5aaa295231f2a9e33bcaf
> Reviewed-on: https://chromium-review.googlesource.com/579567
> Reviewed-by: Dirk Pranke <dpranke@chromium.org>
> Reviewed-by: smut <smut@google.com>
> Commit-Queue: John Budorick <jbudorick@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#488736}

TBR=smut@google.com,dpranke@chromium.org,jbudorick@chromium.org

Change-Id: I7f66b8f8a01d3ea235ba894063e8c0f5897fa4a8
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 740301, 739556
Reviewed-on: https://chromium-review.googlesource.com/582449Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488787}

infra/config/cq.cfg

@@ -67,10 +67,15 @@ verifiers {
     }
     buckets {
       name: "master.tryserver.chromium.mac"
-      builders { name: "ios-device" }
-      builders { name: "ios-device-xcode-clang" }
+      # https://crbug.com/739556; make this non-experimental ASAP.
+      builders {
+        name: "ios-device"
+        experiment_percentage: 100
+      }
+      # https://crbug.com/739556
+      # builders { name: "ios-device-xcode-clang" }
       builders { name: "ios-simulator" }
-      builders { name: "ios-simulator-xcode-clang" }
+      # builders { name: "ios-simulator-xcode-clang" }
       builders { name: "mac_chromium_compile_dbg_ng" }
       builders { name: "mac_chromium_rel_ng" }
     }