[AMS-2025] Delete unused "propose a talk" page

- In 2024 the website didn't have a `/propose` page, instead the homepage linked to PreTalx with the preamble and submission form.
- We certainly don't want people to email us talk proposals - we use PreTalx for voting.
- So, delete the page. (It's not on the main navigation so people can currently only see it if they know it's there, but for completeness.)

Author: issyl0