Remove unused function

[oyvindeide]

Issue
Came across an unused function.

Pre review checklist

• Added appropriate labels

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corresponding labels here: https://github.com/equinor/ert/blob/main/.github/release.yml

[oyvindeide]

test ert please

[codecov-commenter]

Codecov Report

Merging #2910 (e948dce) into main (62af4ed) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2910      +/-   ##
==========================================
+ Coverage   65.08%   65.10%   +0.01%     
==========================================
  Files         649      649              
  Lines       52610    52594      -16     
  Branches     4740     4730      -10     
==========================================
  Hits        34239    34239              
+ Misses      16828    16814      -14     
+ Partials     1543     1541       -2     

Impacted Files	Coverage Δ
libres/lib/enkf/enkf_util.cpp	86.66% <ø> (+30.14%)	⬆️
ert3/console/_errors.py	85.71% <0.00%> (-14.29%)	⬇️
ert3/console/_console.py	87.50% <0.00%> (-0.80%)	⬇️
ert_gui/simulation/run_dialog.py	75.70% <0.00%> (-0.71%)	⬇️
ert3/engine/_run.py	93.20% <0.00%> (ø)
ert/exceptions/__init__.py	100.00% <0.00%> (ø)
ert/exceptions/_exceptions.py	100.00% <0.00%> (ø)
res/enkf/enums/active_mode_enum.py	100.00% <0.00%> (ø)
libres/lib/enkf/enkf_obs.cpp	66.91% <0.00%> (+0.24%)	⬆️
libres/lib/res_util/matrix.cpp	65.35% <0.00%> (+0.27%)	⬆️
... and 3 more

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[jondequinor]

If it is unused, let's nuke it.