Fix factorial issues (#5496)

* updating DIRECTORY.md

* pass integer to `math.factorial` in `project_euler/problem_015`

* remove duplicated factorial function

* updating DIRECTORY.md

* Update maths/factorial_iterative.py

Co-authored-by: Christian Clauss <cclauss@me.com>

* Update factorial_iterative.py

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: Christian Clauss <cclauss@me.com>