Remove legacy WS/SSE code

[Renerick]

Description

I've removed all mentions of WS and SSE from core htmx.js, from the test server and also removed tests

Corresponding issue: 2.0 development 2.0

Probably could use a revision after merging #2025

Testing

I've checked that all tests (excluding removed ones) are 100% parsing. I also manually tested that all extension demos on the test server are working

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded

[Renerick]

After merging updated SSE implementation, it appears that it was a bit more reliant on legacy sse handling of triggers in the core, so I had to clean it up a bit, as well as removing legacy attributes handling from it. I think it's worth it to port some of those clean ups to 1.0 to make extension a bit more stable.