Remove unused cobra.flux_analysis.reaction module

[synchon]

Checklist

• [x] I searched the documentation.
• [x] I looked through similar issues on GitHub.
• [x] I looked up "How to do ... in cobrapy" on a search engine.

Question

The functions of cobra.flux_analysis.reaction module are not used anymore in the code base. Is it used by users or can we remove it? I had the question as the module's functions are not exported via __init__.py .