[Mac os] Remove the multiple dimension swarming for Mac10.9 tests

As chromium 10.9 Mac machines upgrading finished, we need to migrate all
10.9 tests on waterfall to 10.13. Remove the multiple dimension warming
config for Mac10.9 Tests builder.

Bug: 805475
Change-Id: I46df122e2c19a4aa44d8065eebc95aa51d93c85c
Reviewed-on: https://chromium-review.googlesource.com/942234
Commit-Queue: Shenghua Zhang <shenghuazhang@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540668}

testing/buildbot/waterfalls.pyl

@@ -1627,24 +1627,6 @@
           'isolated_scripts': 'chromium_rel_isolated_scripts',
           'scripts': 'chromium_scripts',
         },
-        'swarming': {
-          'dimension_sets': [
-            {
-              "gpu": "none",
-              "cpu": "x86-64",
-              "os": "Mac-10.9",
-              "pool": "Chrome",
-            },
-          ],
-        },
-        'use_multi_dimension_trigger_script': True,
-        'alternate_swarming_dimensions': [
-          {
-            "cpu": "x86-64",
-            "os": "Mac-10.13",
-            "pool": "Chrome",
-          },
-        ],
       },
       'Mac10.9 Tests (dbg)': {
         'test_suites': {