ci: removing linkage-monitor from the required checks (#532)

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

@@ -10,7 +10,6 @@ branchProtectionRules:
  requiredStatusCheckContexts:
  - dependencies (8)
  - dependencies (11)
- - linkage-monitor
  - lint
  - clirr
  - units (8)
@@ -25,7 +24,6 @@ branchProtectionRules:
  requiredStatusCheckContexts:
  - dependencies (8)
  - dependencies (11)
- - linkage-monitor
  - lint
  - clirr
  - units (7)