Bug 1517363 - Rebalance mozmill test chunks, take 2. r=me

taskcluster/ci/test/tests.yml

@@ -83,7 +83,6 @@ mozmill:
         by-test-platform:
             linux32/debug: 8
             linux64/debug: 8
-            macosx64.*: 2
             default: 4
     instance-size: default
     max-run-time: 5400