Revert "Add 10.13 as an optional dimension for layout tests"

This reverts commit 6822d5b7.

Reason for revert: https://crbug.com/908551

Original change's description:
> Add 10.13 as an optional dimension for layout tests
> 
> This will allow them to try 10.13 first and fall back to 10.12 if there
> aren't any hosts available.
> 
> Bug: 853356
> Change-Id: I0fd82376ebb2c4e501a78360bbbc675c90ee7d34
> No-Try: true
> Reviewed-on: https://chromium-review.googlesource.com/c/1330693
> Commit-Queue: Brad Hall <bradhall@google.com>
> Reviewed-by: John Budorick <jbudorick@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#609743}

TBR=jbudorick@chromium.org,bradhall@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 853356
Change-Id: I2d620fd3bec6b43ceda004de3925154a24986d8e
Reviewed-on: https://chromium-review.googlesource.com/c/1351503Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610950}

testing/buildbot/chromium.mac.json

@@ -4537,13 +4537,6 @@
               "os": "Mac-10.12.6"
             }
           ],
-          "optional_dimensions": {
-            "1200": [
-              {
-                "os": "Mac-10.13"
-              }
-            ]
-          },
           "shards": 12
         }
       },

testing/buildbot/test_suite_exceptions.pyl

@@ -1442,6 +1442,7 @@
         },
       },
       'Mac10.13 Tests': {
+        # TODO(crbug.com/853356): Switch this to 10.13.
         # TODO(kbr): if the Swarming dimensions were explicitly specified for
         # all jobs then this wouldn't be needed.
         'swarming': {
@@ -1452,16 +1453,6 @@
             },
           ],
           'shards': 12,
-          # TODO(crbug.com/853356): Switch this fully to 10.13.
-          # Starting with a 20 minute expiration for now, we will reduce this
-          # later.
-          'optional_dimensions': {
-            1200: [
-              {
-                'os': 'Mac-10.13',
-              }
-            ],
-          },
         },
       },
       'Mac10.13 Tests (dbg)': {