Enable PKCE for private clients

[damienbr]

Related issue

#1512
@aeneasr

Proposed changes

Remove the checks in PKCE flow that limits the feature to Public clients only.

Checklist

• I have read the contributing guidelines
• I have read the security policy
• I confirm that this pull request does not address a security vulnerability. If this pull request addresses a security
  vulnerability, I confirm that I got green light (please contact security@ory.sh) from the maintainers to push the changes.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation within the code base (if appropriate)

Further comments

[CLAassistant]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ damienbr
❌ loicvigneron
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[aeneasr]

Awesome - thank you! :)

[nerocrux]

Great! Love this.