remove core requirements from "Mac Builder (reclient)"

fyi_mac_builder sets cores=4 by default, but it requires
bot assignment.
We have some mac builders in pool:luci.chromium.ci with
more cores, so drop core requirements from "Mac Builder (reclient)"

Bug: 1245114
Change-Id: I663e66ab46afa62fa3c2fdbac0d4f1ccbbd99726
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3163836


Auto-Submit: Fumitoshi Ukai <ukai@google.com>
Commit-Queue: Erik Staab <estaab@chromium.org>
Reviewed-by: Gregory Guterman <guterman@google.com>
Reviewed-by: Erik Staab <estaab@chromium.org>
Cr-Commit-Position: refs/heads/main@{#922635}