Disable scheduled Docker workflows

.github/workflows/docker_management.branch.yml

@@ -2,11 +2,6 @@ name: Publish or Update docker image for head of branch
 # Design details in https://github.com/ARMmbed/mbed-os/blob/master/docs/design-documents/docker_management
 
 on:
-
-  # passive update once a week
-  schedule:
-    - cron:  '15 4 * * 6'
-
   # build on master branch when there is changes for active update
   push:
     branches: 

.github/workflows/docker_management.prune.yml

@@ -1,9 +1,6 @@
 name: Prune temporary docker images
 
 on:
-  schedule:
-    - cron:  '15 4 * * 6'
-
   workflow_dispatch:
 
 jobs:

.github/workflows/docker_management.release.yml

@@ -7,9 +7,6 @@ on:
     tags: 
       - mbed-os-6.[0-9]+.[0-9]+
 
-  schedule:
-    - cron:  '15 4 * * 6'
-
   workflow_dispatch:
     inputs:
       mbed_os_release_version: