Use default win10 for win-pixel-tester-rel

This should be a no-op to definition as fleet has moved, but
let's us keep things clean.

Bug: 1124829
Change-Id: I9975c20e5e14bbe510f39acebb5d2e7170d629db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2493098Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Sven Zheng <svenzheng@chromium.org>
Commit-Queue: Ian Struiksma <ianstruiksma@google.com>
Cr-Commit-Position: refs/heads/master@{#820386}

testing/buildbot/waterfalls.pyl

@@ -2820,13 +2820,9 @@
         },
       },
       'win-pixel-tester-rel': {
-        'swarming': {
-          'dimension_sets': [
-            {
-              'os': 'Windows-10-18363',
-            },
-          ],
-        },
+        'mixins': [
+            'win10',
+        ],
         'test_suites': {
           'gtest_tests': 'pixel_browser_tests_gtests',
         }