recursion exercises: refactor: simplify recursion logic in exercises 4 and 5 for learner clarity

[damon314159]

Because

I believe the example solutions provided could do a better job at clearly demonstrating to learners the recursive steps, without additional syntax noise and overly complex logic. See the issue linked below for more detail.

Please note this is not me submitting my solutions as a learner, I am aware of and understand that rule.

This PR

Refactors the example solutions to both those exercises in the way described in my linked issue
All tests still pass

Issue

Closes #670

Pull Request Requirements

• I have thoroughly read and understand The Odin Project Contributing Guide
• The title of this PR follows the location of change: brief description of change format, e.g. 01_helloWorld: Update test cases
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
• If this PR includes any changes that affect the solution of an exercise, I've also updated the solution in the /solutions folder

[mao-sz]

Thanks @damon314159 and for the detailed write-up. I agree on both fronts.

[mao-sz]

🚀