Remove pipeline source limit

Related issue: https://jihulab.com/gitlab-cn/gitlab/-/issues/2715

Remove $CI_PIPELINE_SOURCE == "schedule" limit in .sync, to trigger pipeline through API:

trigger-merge-pre-into-main-jh:
  stage: .post
  rules:
    - if: '$CI_COMMIT_BRANCH == "pre-main-jh"'
  variables:
    # https://jihulab.com/gitlab-cn/code-sync
    SYNC_CODE_PROJECT_ID: 9281
    TRIGGER_CODE_SYNC_PIPELINE_URL: "$CI_SERVER_URL/api/v4/projects/$SYNC_CODE_PROJECT_ID/trigger/pipeline"
  script:
    - echo "Trigger code-sync pipeline to merge pre-main-jh into main-jh"
    - curl -X POST --fail -F token="${TRIGGER_CODE_SYNC_TOKEN}" -F ref="main-jh" -F "variables[JH_SYNC_SCHEDULE]=gitlab-main-jh" $TRIGGER_CODE_SYNC_PIPELINE_URL

https://jihulab.com/gitlab-cn/gitlab/blob/109dbe7010abb8ac8b73dc72c464621a764c6a3e/jh/.gitlab/ci/code-sync.gitlab-ci.yml#L1-L11