Revert of Enable Mac headless tests now that headless targets are inc…

…luded on Mac build. (patchset chromium#2 id:20001 of https://codereview.chromium.org/2783053002/ )

Reason for revert:
These tests are failing on Mac10.10 Tests

https://uberchromegw.corp.google.com/i/chromium.mac/builders/Mac10.10%20Tests

Original issue's description:
> Enable Mac headless tests now that headless targets are included on Mac build.
>
> BUG=687407
>
> Review-Url: https://codereview.chromium.org/2783053002
> Cr-Commit-Position: refs/heads/master@{#461362}
> Committed: https://chromium.googlesource.com/chromium/src/+/69dd94f8ffe4b3d279b599fe4bff3445e20c0c1a

TBR=dpranke@chromium.org,skyostil@chromium.org,dvallet@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=687407

Review-Url: https://codereview.chromium.org/2792893002
Cr-Commit-Position: refs/heads/master@{#461370}

testing/buildbot/chromium.mac.json

@@ -173,28 +173,6 @@
         },
         "test": "gpu_unittests"
       },
-      {
-        "swarming": {
-          "can_use_on_swarming_builders": true
-        },
-        "test": "headless_browsertests"
-      },
-      {
-        "args": [
-          "--enable-browser-side-navigation"
-        ],
-        "name": "browser_side_navigation_headless_browsertests",
-        "swarming": {
-          "can_use_on_swarming_builders": true
-        },
-        "test": "headless_browsertests"
-      },
-      {
-        "swarming": {
-          "can_use_on_swarming_builders": true
-        },
-        "test": "headless_unittests"
-      },
       {
         "swarming": {
           "can_use_on_swarming_builders": false