[core] Remove mocks of require.context in markdown loader

[eps1lon]

Review with "hide whitespace changes" is recommended.

1. remove requireRaw mock
2. remove requireDemo mock (but make sure attempting to render a non-existent demo throws)
3. skip transpiling code created by loader
   This was fairly expensive to do with large markdown files and we don't really need it. We can write es5 code directly instead.

Test plan

• IE 11 "works"
• Localized content SSR and CSR "works": https://60f9833121bff10007deddba--material-ui.netlify.app

[mui-pr-bot]

No bundle size changes (experimental)

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