Remove unused function enkf_main_load_obs

[oyvindeide]

Drive by cleaning

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 #3853 (53c0dcf) into main (431aea8) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3853      +/-   ##
==========================================
+ Coverage   63.86%   63.90%   +0.03%     
==========================================
  Files         595      595              
  Lines       44584    44572      -12     
  Branches     4012     4010       -2     
==========================================
+ Hits        28475    28482       +7     
+ Misses      14869    14850      -19     
  Partials     1240     1240              

Impacted Files	Coverage Δ
src/clib/lib/enkf/enkf_main.cpp	28.84% <ø> (+1.32%)	⬆️
src/ert/ert3/engine/_sensitivity.py	88.63% <0.00%> (-6.37%)	⬇️
src/ert/ert3/engine/_export.py	89.55% <0.00%> (-3.90%)	⬇️
src/ert/gui/ertwidgets/models/ertmodel.py	50.00% <0.00%> (-1.73%)	⬇️
src/ert/ert3/engine/_run.py	95.23% <0.00%> (-1.51%)	⬇️
src/ert/gui/tools/plugins/plugin_handler.py	64.00% <0.00%> (-1.39%)	⬇️
src/ert/gui/ertwidgets/caselist.py	21.91% <0.00%> (-1.06%)	⬇️
...ert/gui/tools/plot/customize/customization_view.py	22.34% <0.00%> (-0.82%)	⬇️
.../config/plugins/implementations/transformations.py	94.28% <0.00%> (-0.46%)	⬇️
src/clib/lib/enkf/gen_obs.cpp	43.80% <0.00%> (-0.33%)	⬇️
... and 169 more

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