MUL311: Remove duplicate operations from statistics-mtg2

[tweska]

Remove de-accumulate, flux-average and fixed-window-flux-average operations. These are replaced by the difference operation and average-rate action.

🌈🌦️📖🚧 Documentation 🚧📖🌦️🌈
https://sites.ecmwf.int/docs/dev-section/multio/pull-requests/PR-198

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.92%. Comparing base (8879ac0) to head (30a1399).
⚠️ Report is 6 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #198      +/-   ##
===========================================
+ Coverage    57.80%   57.92%   +0.11%     
===========================================
  Files          322      317       -5     
  Lines        20567    20509      -58     
  Branches      1604     1594      -10     
===========================================
- Hits         11889    11880       -9     
+ Misses        8678     8629      -49     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.