Tiny fix simplifying the rsr plot code

[adybbroe]

Simplify code, and remove unnecessary argument specifications and typing in the composite_rsr_plot.py script.

(This was a small improvement which was forgotten in PR #240)

• Closes #xxxx
• Tests added
• Tests passed: Passes pytest pyspectral
• Passes flake8 pyspectral
• Fully documented
• Add your name to AUTHORS.md if not there already

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.28%. Comparing base (8ca1082) to head (1660fd4).
Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #246      +/-   ##
==========================================
+ Coverage   90.25%   90.28%   +0.03%     
==========================================
  Files          22       22              
  Lines        2515     2523       +8     
==========================================
+ Hits         2270     2278       +8     
  Misses        245      245              

Flag	Coverage Δ
unittests	90.28% <ø> (+0.03%)	⬆️

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