[docs] Remove notion of seats

[oliviertassinari]

Scale to all the docs: https://github.com/mui/material-ui/pull/43438/files#r1730483431

We don't use it anywhere on https://mui.com/pricing/, https://mui.com/legal/mui-x-eula/

[mui-bot]

Deploy preview: https://deploy-preview-14351--material-ui-x.netlify.app/

Updated pages:

• docs/data/introduction/licensing/licensing.md

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