Remove obsolete requirement files

[daico007]

PR Summary:

I think it's time for us to drop the Pypi remnants. This PR remove all the requirements txt files that used for pip installation.

PR Checklist

---

• Includes appropriate unit test(s)
• Appropriate docstring(s) are added/updated
• Code is (approximately) PEP8 compliant
• Issue(s) raised/addressed?

[codecov]

Codecov Report

Merging #537 (c44c337) into main (8f78f45) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #537   +/-   ##
=======================================
  Coverage   69.18%   69.18%           
=======================================
  Files          16       16           
  Lines        1671     1671           
=======================================
  Hits         1156     1156           
  Misses        515      515