Remove ert3

[eivindjahren]

Removes ert3. Bye bye for now 😢

The test have passed, but the required ert3 tests will have to be removed from the required list.

Pre review checklist

• Added appropriate release note label
• PR title captures the intent of the changes, and is fitting for release notes.
• Commit history is consistent and clean, in line with the contribution guidelines.

Adding labels helps the maintainers when writing release notes. This is the list of release note labels.

[codecov-commenter]

Codecov Report

Merging #3877 (e95c0e8) into main (52ce69c) will decrease coverage by 2.57%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3877      +/-   ##
==========================================
- Coverage   63.74%   61.16%   -2.58%     
==========================================
  Files         588      551      -37     
  Lines       43841    42003    -1838     
  Branches     3786     3790       +4     
==========================================
- Hits        27946    25691    -2255     
- Misses      14772    15187     +415     
- Partials     1123     1125       +2     

Impacted Files	Coverage Δ
...rt/ensemble_evaluator/tracker/evaluator_tracker.py	72.00% <ø> (-0.23%)	⬇️
src/ert/gui/simulation/run_dialog.py	81.15% <ø> (+1.34%)	⬆️
src/ert/storage/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
src/ert/storage/_storage.py	0.00% <0.00%> (-89.46%)	⬇️
src/ert/data/record/_transmitter.py	56.66% <0.00%> (-37.78%)	⬇️
src/ert/data/record/_record.py	62.89% <0.00%> (-30.32%)	⬇️
src/ert/exceptions/_exceptions.py	70.96% <0.00%> (-29.04%)	⬇️
src/ert/data/record/_transformation.py	63.80% <0.00%> (-24.77%)	⬇️
src/ert/shared/services/storage_service.py	55.81% <0.00%> (-20.94%)	⬇️
src/clib/lib/job_queue/environment_varlist.cpp	13.15% <0.00%> (-16.26%)	⬇️
... and 15 more

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[sondreso]

LGTM! Farewell for now ert3 👋 😕