Move win10-blink-rel-dummy to use default win10 mixin.

This causes no change in generated json, but allows the migration
to newer win10 version to be more straightforward.

Bug: 1124829
Change-Id: I483eedee77cb247d81fd32ec874675f53c26b18d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2488679
Auto-Submit: Ian Struiksma <ianstruiksma@google.com>
Commit-Queue: Erik Staab <estaab@chromium.org>
Reviewed-by: Erik Staab <estaab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819144}

testing/buildbot/waterfalls.pyl

@@ -2839,12 +2839,10 @@
         'use_swarming': False,
       },
       'win10-blink-rel-dummy': {
+        'mixins': [
+            'win10',
+        ],
         'swarming': {
-          'dimension_sets': [
-            {
-              'os': 'Windows-10-15063',
-            },
-          ],
           'hard_timeout': 900,
         },
         'test_suites': {