Remove unused keyword ENKF_MODE

[andreas-el]

Remove keyword from documentation
Add keyword to just-remove-list

Keyword has not been used last three months, and functionality was removed back in 2019.

Issue
Resolves #my_issue

Approach
Short description of the approach

(Screenshot of new behavior in GUI if applicable)

Pre review checklist

• Read through the code changes carefully after finishing work
• Make sure tests pass locally (after every commit!)
• Prepare changes in small commits for more convenient review (optional)
• PR title captures the intent of the changes, and is fitting for release notes.
• Updated documentation
• Ensured that unit tests are added for all new behavior (See
  Ground Rules),
  and changes to existing code have good test coverage.

Pre merge checklist

• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9d1b2fb) 83.51% compared to head (eff5519) 83.51%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6568   +/-   ##
=======================================
  Coverage   83.51%   83.51%           
=======================================
  Files         346      346           
  Lines       20764    20764           
  Branches      948      948           
=======================================
  Hits        17341    17341           
  Misses       3129     3129           
  Partials      294      294           

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[DanSava]

Nice find 🪐