Revert "Default-on parallel push checks"

This reverts commit 2dca4d37.

ee/config/feature_flags/ops/parallel_push_checks.yml

@@ -5,4 +5,4 @@ rollout_issue_url:
 milestone: '13.6'
 type: ops
 group: group::source_code
-default_enabled: true
+default_enabled: false

ee/lib/ee/gitlab/checks/push_rule_check.rb

@@ -7,7 +7,7 @@ module EE
         def validate!
           return unless push_rule
 
-          if ::Feature.enabled?(:parallel_push_checks, project, type: :ops, default_enabled: true)
+          if ::Feature.enabled?(:parallel_push_checks, project, type: :ops)
             run_checks_in_parallel!
           else
             run_checks_in_sequence!