Temporarily remove macOS from test matrix (#20194)

Author: pakrym

eng/pipelines/templates/stages/platform-matrix.json

@@ -12,10 +12,6 @@
       "windows-2019": {
           "OSVmImage": "MMS2019",
           "Pool": "azsdk-pool-mms-win-2019-general"
-      },
-      "macOS-10.15": {
-          "OSVmImage": "macOS-10.15",
-          "Pool": "Azure Pipelines"
       }
     },
     "TestTargetFramework": [