Revert "Linux ARM: Use swarming mixins"

This reverts commit 9fc367df.

Reason for revert: Broke bot

Original change's description:
> Linux ARM: Use swarming mixins
> 
> This change is deliberately small (only changing one builder) to make
> sure the feature is working correctly.
> 
> Bug: 880973
> Change-Id: If9ffea3eadc461a994cd7dcc409b9b8206871024
> Reviewed-on: https://chromium-review.googlesource.com/1222165
> Reviewed-by: Kenneth Russell <kbr@chromium.org>
> Commit-Queue: Stephen Martinis <martiniss@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#591241}

TBR=dpranke@chromium.org,kbr@chromium.org,martiniss@chromium.org,bpastene@chromium.org,jbudorick@chromium.org

Change-Id: I0aca3b5b53c5f74f0af8f5fc287051cc4457cdd2
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 880973
Reviewed-on: https://chromium-review.googlesource.com/1227394Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591484}

testing/buildbot/chromium.fyi.json

@@ -947,25 +947,13 @@
     "gtest_tests": [
       {
         "swarming": {
-          "can_use_on_swarming_builders": true,
-          "dimension_sets": [
-            {
-              "cpu": "arm7l-32",
-              "os": "Ubuntu-14.04"
-            }
-          ]
+          "can_use_on_swarming_builders": true
         },
         "test": "nacl_helper_nonsfi_unittests"
       },
       {
         "swarming": {
-          "can_use_on_swarming_builders": true,
-          "dimension_sets": [
-            {
-              "cpu": "arm7l-32",
-              "os": "Ubuntu-14.04"
-            }
-          ]
+          "can_use_on_swarming_builders": true
         },
         "test": "nacl_loader_unittests"
       },
@@ -974,13 +962,7 @@
           "--test-launcher-print-test-stdio=always"
         ],
         "swarming": {
-          "can_use_on_swarming_builders": true,
-          "dimension_sets": [
-            {
-              "cpu": "arm7l-32",
-              "os": "Ubuntu-14.04"
-            }
-          ]
+          "can_use_on_swarming_builders": true
         },
         "test": "sandbox_linux_unittests"
       }

testing/buildbot/swarming_mixins.pyl

@@ -15,14 +15,4 @@
 # already exists.
 
 {
-  'linux-trusty': {
-    'dimensions': {
-      'os': 'Ubuntu-14.04',
-    }
-  },
-  'arm_tester': {
-    'dimensions': {
-      'cpu': 'arm7l-32',
-    }
-  }
 }

testing/buildbot/waterfalls.pyl

@@ -2432,10 +2432,6 @@
         ],
       },
       'Linux ARM': {
-        'swarming_mixins': [
-          'linux-trusty',
-          'arm_tester',
-        ],
         'test_suites': {
           'gtest_tests': 'linux_arm_gtests',
         },