Remove MacOS from platform support testing matrix

MacOS is not an officially supported production platform. Maintaining
this test infrastructure is costly and has historically been very
unstable. While we won't maintain continuous integration here, we'll
still continue to build and publish MacOS artifacts and support it in
developoment environments.

.ci/jobs.t/elastic+elasticsearch+periodic+platform-support.yml

@@ -13,10 +13,6 @@
       - multijob:
           name: Packaging tests
           projects:
-            - name: elastic+elasticsearch+%BRANCH%+multijob+platform-support-darwin
-              kill-phase-on: NEVER
-              current-parameters: true
-              git-revision: true
             - name: elastic+elasticsearch+%BRANCH%+multijob+platform-support-unix
               kill-phase-on: NEVER
               current-parameters: true