Revert combination of datasets

[oyvindeide]

This is being reverted because the checking of the state takes too long, causing the start up time for ert to become too slow.

This blocks stable releases, so will be reverted to unblock that, and can be reinstated at a later time.

This revert commit will slow down the plotting significantly, as that was the main benefit of having unified datasets.

• [x] 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 tests pass locally (after every commit!)

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: Create Backport PR to latest release

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 92.93893% with 37 lines in your changes missing coverage. Please review.

Project coverage is 86.89%. Comparing base (5105f4a) to head (accb736).

Files	Patch %	Lines
src/ert/config/observations.py	93.43%	13 Missing :warning:
src/ert/storage/local_ensemble.py	92.95%	5 Missing :warning:
src/ert/libres_facade.py	33.33%	4 Missing :warning:
src/ert/config/observation_vector.py	90.62%	3 Missing :warning:
src/ert/dark_storage/common.py	93.02%	3 Missing :warning:
src/ert/dark_storage/endpoints/records.py	81.25%	3 Missing :warning:
src/ert/data/_measured_data.py	91.66%	3 Missing :warning:
src/ert/analysis/_es_update.py	93.93%	2 Missing :warning:
src/ert/config/ensemble_config.py	95.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #8247 +/- ## ========================================== - Coverage 87.03% 86.89% -0.14% ========================================== Files 378 375 -3 Lines 23953 23561 -392 Branches 618 619 +1 ========================================== - Hits 20847 20473 -374 + Misses 3025 3007 -18 Partials 81 81 ```

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