chore: stop requiring linkage-monitor check (googleapis#738)

Author: chingor13

.github/sync-repo-settings.yaml

@@ -7,7 +7,6 @@ branchProtectionRules:
     requiredStatusCheckContexts:
       - dependencies (8)
       - dependencies (11)
-      - linkage-monitor
       - lint
       - clirr
       - units (8)
@@ -21,7 +20,6 @@ branchProtectionRules:
     requiredStatusCheckContexts:
       - dependencies (8)
       - dependencies (11)
-      - linkage-monitor
       - lint
       - clirr
       - units (7)
@@ -36,7 +34,6 @@ branchProtectionRules:
     requiredStatusCheckContexts:
       - dependencies (8)
       - dependencies (11)
-      - linkage-monitor
       - lint
       - clirr
       - units (8)