docs: remove futureEmitAssets

[alan-agius4]

This is no longer present in version 5

• Read and sign the CLA. PRs that haven't signed it won't be accepted.
• Make sure your PR complies with the writer's guide.
• Review the diff carefully as sometimes this can reveal issues.
• Do not abandon your Pull Request: Stale Pull Requests.
• Remove these instructions from your PR as they are for your eyes only.

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[chenxsan]

Indeed, I can't find any related code in webpack 5 repository now. Thanks!