mac fyi bot: drop explicit cores requirement

Looks like this is one of two issues causing the current query
to not find the assigned machine:

https://chromium-swarm.appspot.com/task?id=4cffe1b5410d6110&o=true&w=true

TBR=mark@chromium.org

Bug: 1098454
Change-Id: If294766c1e94e1dcfd9d7b0b99f148cf8d8dff0b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2264260
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781915}

infra/config/generated/cr-buildbucket.cfg

@@ -11749,7 +11749,6 @@ buckets {
       swarming_host: "chromium-swarm.appspot.com"
       swarming_tags: "vpython:native-python-wrapper"
       dimensions: "builder:mac-arm64"
-      dimensions: "cores:8"
       dimensions: "cpu:x86-64"
       dimensions: "os:Mac"
       dimensions: "pool:luci.chromium.ci"

infra/config/subprojects/chromium/ci.star

@@ -666,6 +666,7 @@ ci.fyi_builder(
         short_name = 'arm64',
     ),
     main_console_view = None,
+    cores = None,
     os = os.MAC_ANY,
 )