gh-38045: simplify handling of dead oeis sequences
Simplify the handling of dead OEIS sequences. Trying to ignore them,
mostly.
This fixes #38044
### :memo: Checklist
- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [x] I have updated the documentation and checked the documentation
preview.
URL: https://github.com/sagemath/sage/pull/38045
Reported by: Frédéric Chapoton
Reviewer(s):
