CI coverage config references removed modules, misses feature_elimination

[detrin]

Hey! Noticed the CI workflow (unit_tests.yml) still references four modules that were removed in v3.0.0:

--cov=probatus/binning --cov=probatus/metric_volatility --cov=probatus/missing_values --cov=probatus/stat_tests

None of these exist anymore. More importantly, probatus/feature_elimination - the largest module at ~1,350 LOC - isn't in the coverage list at all, so Codecov has been blind to it.

Quick fix: replace the stale paths with --cov=probatus/feature_elimination and drop the dead ones.