remove ability for +page.server.js to respond to GET requests with JSON

[Rich-Harris]

Closes #5936. This behaviour is nothing more than a historical accident. It's buggy, and there's no reason to keep it.

If we want routes to serve both HTML and non-HTML, #5896 is a more promising approach.

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[benmccann]

I wonder if it's worth implementing #5896 as part of this same PR. It looks like you might be able to leave a couple of the tests in place then

[Rich-Harris]

I think we'll want separate tests. I didn't want to do it in this PR because it's probably going to be a chunk of work