Don't explicitly request 4 CPUs for mac libfuzzer luci bot.

Its pool currently has bots w/ a mix of core counts:
https://chromium-swarm.appspot.com/oldui/botlist?c=id&c=os&c=task&c=status&c=cores&f=builder%3ALibfuzzer%20Upload%20Mac%20ASan&l=100&q=cores&s=id%3Aasc

R=jbudorick

Bug: 790372
Change-Id: I0b5194a170c0e62b7768a0f7f05b6c4dde63b803
Reviewed-on: https://chromium-review.googlesource.com/c/1357526Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612842}

infra/config/global/cr-buildbucket.cfg

@@ -2017,7 +2017,6 @@ buckets {
     builders {
       name: "Libfuzzer Upload Mac ASan"
       dimensions: "os:Mac-10.13"
-      dimensions: "cores:4"
       mixins: "fyi-ci"
       # mixins: "fuzz-ci"
       mixins: "libfuzzer"