ProcessedVariables: fix 2D scikit FEM bug

[MarcBerliner]

Description

The 2D scikit FEM observables were missing a transpose.

This PR also removes unused Python observable code, which was replaced by C++ code.

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #)

Important checks:

Please confirm the following before marking the PR as ready for review:

• No style issues: nox -s pre-commit
• All tests pass: nox -s tests
• The documentation builds: nox -s doctests
• Code is commented for hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.71%. Comparing base (6ad2495) to head (b2f05bc).
Report is 75 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4912      +/-   ##
===========================================
- Coverage    98.71%   98.71%   -0.01%     
===========================================
  Files          304      304              
  Lines        23509    23478      -31     
===========================================
- Hits         23207    23176      -31     
  Misses         302      302              

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