Remove endpoints only used by webviz-ert

[frode-aarstad]

Also remove some unused code and old webviz-ert testing

Issue
Resolves equinor/webviz-ert#503

Approach
Short description of the approach

(Screenshot of new behavior in GUI if applicable)

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'just rapid-tests')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Add backport label to latest release (format: 'backport release-branch-name')

[codspeed-hq]

Merging this PR will not alter performance

✅ 35 untouched benchmarks

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_{Comparing frode-aarstad:remove-webviz-ert-entries (583ac25) with main (dd7b38c)}

[xjules]

This needs to stay for the obs in field visualization

[xjules]

We need to discuss this first as the observation endpoints I'm planning to use for some localization related visualization (see. #12958).
Moreover, I thought sumo wants to be a consumer of observations too.