Don't request an os dimension for Mac Builder Next.

Mac Builder Next doesn't use a builderless machine; there's one machine
that is allocated to it. When the machine was updated to Mac 10.15, its
tasks started expiring because the machine no longer matched the os
dimension.

Change-Id: I039c61f381d0a65204a62b9c1333b1f6b652a416
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2083561
Auto-Submit: Garrett Beaty <gbeaty@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746449}

infra/config/buckets/ci.star

@@ -1186,7 +1186,7 @@ ci.fyi_ios_builder(
 ci.fyi_mac_builder(
     name = 'Mac Builder Next',
     cores = None,
-    os = os.MAC_10_14,
+    os = None,
 )
 
 ci.fyi_mac_builder(

infra/config/generated/cr-buildbucket.cfg

@@ -3452,7 +3452,6 @@ buckets: <
       swarming_tags: "vpython:native-python-wrapper"
       dimensions: "builder:Mac Builder Next"
       dimensions: "cpu:x86-64"
-      dimensions: "os:Mac-10.14"
       recipe: <
         name: "chromium"
         cipd_package: "infra/recipe_bundles/chromium.googlesource.com/chromium/tools/build"