Revert "Enable DCHECKs on amd64-generic simplechrome CQ bot."

This reverts commit 4dd5af99.

Reason for revert: Browser crashes flakily with DCHECKs on

See
https://chromium-swarm.appspot.com/task?id=3f063cbec4a26e10
https://chromium-swarm.appspot.com/task?id=3f063760c6162910
https://chromium-swarm.appspot.com/task?id=3f063164005d9710

Example crashes
https://isolateserver.appspot.com/browse?namespace=default-gzip&digest=e4fef843d0fef24b39d59d06bfddbe1569402f2b&as=chrome.PREVIOUS
https://isolateserver.appspot.com/browse?namespace=default-gzip&digest=4d98a300e399d97b4b02cf89f01134050e2f985e&as=chrome_20180730-160158
https://isolateserver.appspot.com/browse?namespace=default-gzip&digest=a4068c05476b786213ab26a80cf35e426fd54f44&as=chrome_20180730-155114

Original change's description:
> Enable DCHECKs on amd64-generic simplechrome CQ bot.
> 
> Bug: 867077
> Change-Id: Icad4a62735d3be6579b3c6476bbbb65ad1843ec8
> Reviewed-on: https://chromium-review.googlesource.com/1150939
> Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
> Reviewed-by: John Budorick <jbudorick@chromium.org>
> Reviewed-by: Dirk Pranke <dpranke@chromium.org>
> Commit-Queue: Ben Pastene <bpastene@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#579118}

TBR=dpranke@chromium.org,dcastagna@chromium.org,bpastene@chromium.org,jbudorick@chromium.org

Change-Id: I42e28d289cd2c6604b829b9971abb0783fa5ee76
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 867077
Reviewed-on: https://chromium-review.googlesource.com/1155500Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579231}

tools/mb/mb_config.pyl

@@ -560,7 +560,7 @@
     },
 
     'tryserver.chromium.chromiumos': {
-      'chromeos-amd64-generic-rel': 'cros_chrome_sdk_dcheck_always_on',
+      'chromeos-amd64-generic-rel': 'cros_chrome_sdk',
       'chromeos-daisy-rel': 'cros_chrome_sdk',
       'linux-chromeos-rel': 'chromeos_with_codecs_release_trybot',
       'linux-chromeos-compile-dbg': 'chromeos_with_codecs_debug_bot',
@@ -1174,10 +1174,6 @@
       'cros_chrome_sdk',
     ],
 
-    'cros_chrome_sdk_dcheck_always_on': [
-      'cros_chrome_sdk', 'dcheck_always_on',
-    ],
-
     'cros_chrome_sdk_headless_ozone': [
       'cros_chrome_sdk', 'ozone_platform_headless',
     ],