Use built-in option to remove padding

[dee-see]

Summary

I was reading the code and came across how the padding is stripped from the code_challenge. There's a builtin option in Base64.urlsafe_encode64(..., padding: false) to do that. Tiny tiny change but we're saving a split. :)

[doorkeeper-bot]

	2 Warnings
⚠️	There're library changes, but not tests. That's OK as long as you're refactoring existing code.
⚠️	Please include a changelog entry. You can find it at CHANGELOG.md.You can skip this warning only if you made some typo fix or other small changes that didn't affect the API.

Here's an example of a CHANGELOG.md entry:

- [#1459] Use built-in option to remove padding.

Generated by 🚫 Danger

[dee-see]

I think we can skip the changelog for this?

[nbulaj]

You don't know @dee-see how I love such changes 👍
Thanks! Yeah, no need in changelog I think