Remove unused CSS rules from completion-modal.css

[huyenltnguyen]

Description

The completion-modal.css contains a lot of unused CSS rules, which should be cleaned up.

The unused rules/classes are:

• .challenge-success-modal
• .btn-lg
• .btn-cta-big
• .completion-modal-body
• .completion-message
• .completion-challenge-name

Requirements

• Remove the above classes from https://github.com/freeCodeCamp/freeCodeCamp/blob/main/client/src/templates/Challenges/components/completion-modal.css
• Test and confirm that the modal displays properly after the changes

[MyBaBB]

Hi, I can help with this If you would like to assign it to me.