Simplify Experiment code after dropping RLExperiment

[jeremiahpslewis]

Environment string parsing is complicated, of somewhat limited use and now much less relevant without RLExperiments
PR Checklist

• Update NEWS.md?
• Unit tests for all structs / functions?
• Integration and correctness tests using a simple env?
• PR Review?
• Add or update documentation?
• Write docstrings for new methods?

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 23.98%. Comparing base (c821bb6) to head (5c07360).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1044      +/-   ##
==========================================
+ Coverage   23.78%   23.98%   +0.19%     
==========================================
  Files         231      231              
  Lines        7983     7956      -27     
==========================================
+ Hits         1899     1908       +9     
+ Misses       6084     6048      -36     

Files	Coverage Δ
src/ReinforcementLearningCore/src/core/run.jl	94.73% <50.00%> (+45.50%)	⬆️

... and 2 files with indirect coverage changes

[HenriDeh]

This is such a good step in the direction of simplification.