remove models that don't support generic summaries.

[njbrake]

What's changing

Remove the two models that don't perform well on generic summarization tasks.

If this PR is related to an issue or closes one, please link it here.
Closes #621

How to test it

Steps to test the changes:

1. make local-up
2. Open lumigator, add dataset, run experiment, observe that that mikeadimech/longformer-qmsum-meeting-summarization and mrm8488/t5-base-finetuned-summarize-news are gone.

Additional notes for reviewers

I already...

• [ x] Tested the changes in a working environment to ensure they work as expected
• Added some tests for any new functionality
• [x ] Updated the documentation (both comments in code and product documentation under /docs)
• [x ] Checked if a (backend) DB migration step was required and included it if required

[veekaybee]

Did a git grep and checked for these, looks great, thanks!