ci: removing linkage-monitor from the required checks (#575)

Linkage Monitor is no longer needed, because the Libraries BOM synchronizes with Google Cloud BOM and the shared dependencies BOM GoogleCloudPlatform/cloud-opensource-java#2137

Author: suztomo

.github/sync-repo-settings.yaml

@@ -31,7 +31,6 @@ branchProtectionRules:
   requiredStatusCheckContexts:
     - "dependencies (8)"
     - "dependencies (11)"
-    - "linkage-monitor"
     - "lint"
     - "clirr"
     - "units (8)"
@@ -57,7 +56,6 @@ branchProtectionRules:
   requiredStatusCheckContexts:
     - "dependencies (8)"
     - "dependencies (11)"
-    - "linkage-monitor"
     - "lint"
     - "clirr"
     - "units (7)"