Remove support for custom responses

[jplhomer]

Description

Fixes #1267.

The concept of "custom responses" was introduced early in Hydrogen's development, prior to API routes. However, we haven't seen this functionality used beyond simple sitemap.xml or robots.txt purposes — both of which can be handled by API routes.

This PR removes response.send() as well as the custom body, which only existed to support custom responses.

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Before submitting the PR, please make sure you do the following:

• Read the Contributing Guidelines
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123)
• Update docs in this repository according to your change
• Run yarn changeset add if this PR cause a version bump based on Keep a Changelog and adheres to Semantic Versioning

[jplhomer]

Note that, unless the developer provides a defaultLanguageCode in their hydrogen.config.js, we default to 'EN' anyway. If the developer wishes to customize language code based on e.g. domain, they can add functionality here to do so, since it's part of the demo store template code.

[mcvinci]

Thanks for removing the docs around this as well, @jplhomer!

[mcvinci]

Could we also update framework/seo.md which has references to sitemap.xml.server and robots.txt.server being a JSX files (now JS files)?

[frandiox]

LGTM! We can also make some existing functions like onAllReady or prepareForStreaming synchronous now that this code is removed.