Remove WMO mapping (and scaling) for snow cover

[dsarmany]

As an exception to the general rule, units of both parameters are WMO-compliant

Description

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

🌈🌦️📖🚧 Documentation 🚧📖🌦️🌈
https://sites.ecmwf.int/docs/dev-section/multio/pull-requests/PR-255

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 56.34%. Comparing base (bf1874f) to head (f51fa95).

Files with missing lines	Patch %	Lines
src/multio/mars2mars/Rules.cc	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #255      +/-   ##
===========================================
+ Coverage    56.32%   56.34%   +0.01%     
===========================================
  Files          321      321              
  Lines        21152    21148       -4     
  Branches      1680     1682       +2     
===========================================
+ Hits         11914    11915       +1     
+ Misses        9238     9233       -5     

☔ 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.