Remove 10.13 OR for mac_default

Bug: 1121185
Change-Id: I7f4d0a0517f7286c120c0dd114d48da90488bd17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2493047Reviewed-by: Erik Staab <estaab@chromium.org>
Commit-Queue: Lindsay Pasricha <lindsayw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820313}

infra/config/generated/cr-buildbucket-dev.cfg

@@ -152,7 +152,7 @@ buckets {
       swarming_host: "chromium-swarm-dev.appspot.com"
       swarming_tags: "vpython:native-python-wrapper"
       dimensions: "cpu:x86-64"
-      dimensions: "os:Mac-10.13|Mac-10.15"
+      dimensions: "os:Mac-10.15"
       recipe {
         name: "swarming/staging"
         cipd_package: "infra/recipe_bundles/chromium.googlesource.com/chromium/tools/build"

infra/config/lib/builders.star

@@ -65,8 +65,7 @@ os = struct(
     MAC_10_14 = os_enum("Mac-10.14", os_category.MAC),
     MAC_10_15 = os_enum("Mac-10.15", os_category.MAC),
     MAC_11_0 = os_enum("Mac-11.0", os_category.MAC),
-    # TODO(crbug/1121185): Remove 10.13 once builders have been migrated to 10.15.
-    MAC_DEFAULT = os_enum("Mac-10.13|Mac-10.15", os_category.MAC),
+    MAC_DEFAULT = os_enum("Mac-10.15", os_category.MAC),
     MAC_ANY = os_enum("Mac", os_category.MAC),
     WINDOWS_7 = os_enum("Windows-7", os_category.WINDOWS),
     WINDOWS_8_1 = os_enum("Windows-8.1", os_category.WINDOWS),