Clean up stale mock parameters and dead code

[rcannood]

Describe your changes

Housekeeping picked up while reading through the repo. No behaviour changes.

• _viash.yaml: info.image was still the literal template placeholder ("The name of the image file to use for the component on the website."). Points at thumbnail.svg now.

• Three ## VIASH START blocks had drifted so far that the scripts can't be run locally any more:

  • data_processors/process_dataset still declares output_validation and never declares output_unintegrated.
  • methods/gaussnorm has temp_dir: '/tmp' -- YAML syntax inside an R list(), so the block doesn't even parse -- plus a resources_test/.../cyto_spleen_subset/ path that no longer exists and no resources_dir.
  • control_methods/shuffle_integration_within_batch still declares a single output and no resources_dir, while the script writes output_integrated_split1/2 and does sys.path.append(meta["resources_dir"]).

  Viash replaces these blocks at build time, so none of this affects the built components -- it just makes the scripts annoying to debug by hand.

• Dropped subset_by_celltype() from src/utils/helper_functions.R. Nothing calls it, and it only returns its result by accident (via the value of the assignment in the if/else).

• Dropped the unused concat_paired_samples/fit_r2 imports and a ### Debugging line, can be removed later print of the full obs from metrics/average_batch_r2, and made the log line say "per cell type" rather than "global".

I left the CHANGELOG entries that mention components which no longer exist (methods/mnn, metrics/cms, metrics/emd_per_samples, control_methods/shuffle_integration) alone -- rewriting history entries felt like your call rather than mine. Happy to add explicit removal entries if you'd prefer that.

Found while reviewing the task ahead of the next full benchmark run -- see also the sibling PRs. Note that all of these PRs append to the bottom of CHANGELOG.md, so whichever ones land after the first will need a one-line rebase there.

Checklist before requesting a review

• I have performed a self-review of my code

• Check the correct box. Does this PR contain:

  • Breaking changes
  • New functionality
  • Major changes
  • Minor changes
  • Bug fixes

• Proposed changes are described in the CHANGELOG.md

• CI Tests succeed and look good!

[rcannood]

This PR is trivial enough to merge directly