Remove unnecessary if in the user guide workflow

[jneira]

As github already is not triggering workflows for commits with skip ci as demonstrated here: #6200

[andreasabel]

The official reference seems to be this: https://github.blog/changelog/2021-02-08-github-actions-skip-pull-request-and-push-workflows-with-skip-ci/

GitHub Actions now supports skipping push and pull_request workflows by looking for some common keywords in your commit message.
If any commit message in your push or the HEAD commit of your PR contains the strings [skip ci], [ci skip], [no ci], [skip actions], or [actions skip] workflows triggered on the push or pull_request events will be skipped.