breaking: Delete rawBody handling on /symbolicate endpoint

[huntie]

Summary

This removes a longstanding hack that covered for a lack of JSON body parsing in Metro's /symbolicate endpoint. We're now resolving this in facebook/metro#1475 — meaning this can be removed from CLI.

Relevant changes:

• Add Content-Type header to dev server requests facebook/react-native#49042

Breaking: CLI will no longer intercept and populate req.rawBody on /symbolicate requests. Clients MUST send the header 'Content-Type': 'application/json'.

Test Plan

• Build this repo via yarn build.
• Link @react-native-community/cli-tools and @react-native-community/cli-server-api into a local checkout of react-native via yarn link.
• Open RNTester, modified to throw an error on launch.

TODO

Checklist

• Documentation is up to date.
• Follows commit message convention described in CONTRIBUTING.md.
• For functional changes, my test plan has linked these CLI changes into a local react-native checkout (instructions).