chore: remove non-custom rules from sync-repo-settings (#241)

See https://github.com/googleapis/repo-automation-bots/blob/63c858e539e1f4d9bb8ea66e12f9c0a0de5fef55/packages/sync-repo-settings/src/required-checks.json#L40-L50 for defaults.

Author: busunkim96

.github/sync-repo-settings.yaml

@@ -1,12 +1,3 @@
-rebaseMergeAllowed: true
-squashMergeAllowed: true
-mergeCommitAllowed: false
-branchProtectionRules:
-- pattern: master
-  isAdminEnforced: true
-  requiredApprovingReviewCount: 1
-  requiresCodeOwnerReviews: true
-  requiresStrictStatusChecks: true
 permissionRules:
   - team: actools-python
     permission: admin