remove resuming ability, keep resume config but complain if true

[jbloomAus]

Description

I'm removing the ability to resume a previous run, but keeping the "save SAE on ctrl c" functionality which came with that features. As with SAE dictionaries, this is about cleaning up the code and making room for cleaner code. I realise that this regression might make it hard to work with compute that may be revoked mid-job. However, this is not a use case we intend to support in the short - medium term.

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

Checklist:

• [x] I have commented my code, particularly in hard-to-understand areas
• [] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [] I have not rewritten tests relating to key interfaces which would affect backward compatibility

You have tested formatting, typing and unit tests (acceptance tests not currently in use)

• [x] I have run make check-ci to check format and linting. (you can run make format to format code if needed.)

Performance Check. (not relevant)

If you have implemented a training change, please indicate precisely how performance changes with respect to the following metrics:

• [ ] L0
• [ ] CE Loss
• [ ] MSE Loss
• [ ] Feature Dashboard Interpretability

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 43.47826% with 13 lines in your changes are missing coverage. Please review.

Project coverage is 67.27%. Comparing base (91239c1) to head (b3b28f6).

Files	Patch %	Lines
sae_lens/training/lm_runner.py	23.52%	13 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #156 +/- ## ========================================== - Coverage 71.62% 67.27% -4.35% ========================================== Files 18 18 Lines 1815 1794 -21 Branches 287 282 -5 ========================================== - Hits 1300 1207 -93 - Misses 448 522 +74 + Partials 67 65 -2 ```

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