Remove all macOS-10.15 related code (#7078)

images.CI/macos/azure-pipelines/macos1015.yml

@@ -1,23 +0,0 @@
-name: macOS-10.15_$(date:yyyyMMdd)$(rev:.r)_unstable
-schedules:
-- cron: "0 0 * * *"
-  displayName: Daily
-  branches:
-    include:
-    - main
-  always: true
-
-trigger: none
-pr:
-  autoCancel: true
-  branches:
-    include:
-    - main
-
-jobs:
-- template: image-generation.yml
-  parameters:
-    image_label: 'macOS Catalina'
-    base_image_name: 'clean-macOS-10.15-380Gb-runner'
-    template_path: 'templates/macOS-10.15.json'
-    target_datastore: 'ds-image'