Remove obsolete npm command + fix links to ws/sse extensions

[Telroshan]

Follow-up to #3052 , when we moved the ws/sse test suite to the extensions repo, we forgot some obsolete links/mentions behind

[lkollar]

There is another mention of the /test/ws-sse folder here in the docs: https://github.com/bigskysoftware/htmx/blob/master/www/content/extensions/ws.md#testing-with-the-demo-server.

[Telroshan]

@lkollar thanks, though this one already links to the extensions repo where /test/ws-sse is, so I'd say it is correct as is!

It is located in the /test/ws-sse folder of the htmx-extensions repository. Look at /test/ws-sse/README.md for instructions on running and using the test server.

[lkollar]

Oh don't mind me then. I probably had an old checkout as I missed the link to the extensions repo.