fix(guide): removed guide set-related articles no longer needed

[RandellDawson]

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• My pull request has a descriptive title (not a vague title like Update index.md)
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Since PR #34717 was merged recently which combined three set-related curriculum challenges into one, the following challenge files were deleted:

curriculum/challenges/english/08-coding-interview-prep/data-structures/remove-from-a-set.english.md
curriculum/challenges/english/08-coding-interview-prep/data-structures/size-of-the-set.english.md

This PR removes the corresponding Guide articles related to the deleted challenges.

guide/english/certifications/coding-interview-prep/data-structures/remove-from-a-set/index.md
guide/english/certifications/coding-interview-prep/data-structures/size-of-the-set/index.md

[RandellDawson]

Attention Reviewers: This PR can not be merged before #34946, because it deletes files which are on that PR and would cause a merge conflict.

[RandellDawson]

#34946 has been merged, so this one can be safely merged once approved.

[moT01]

Looks good @RandellDawson! 🎉