[docs] Remove generated CSS utility component doc

[mj12albert]

All the direct system props were removed in v9, the cssComponent option for the docs generator is obsolete now

Closes #48382

• I have followed (at least) the PR section of the contributing guide.

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@mui/material	0B(0.00%)	0B(0.00%)
@mui/lab	0B(0.00%)	0B(0.00%)
@mui/private-theming	0B(0.00%)	0B(0.00%)
@mui/system	0B(0.00%)	0B(0.00%)
@mui/utils	0B(0.00%)	0B(0.00%)

Details of bundle changes

Deploy preview

• docs/data/system/components/box/box.md
• docs/data/system/getting-started/usage/usage.md

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[LukasTy]

Looks nice. 👍
A couple Codex suggestions:

1. Medium: docs/data/system/getting-started/usage/usage.md#L165-L174 still tells users that Box, Stack, Typography, and Grid “accept MUI System properties as props”. That directly contradicts both this PR’s rationale and the existing v9 migration guide at docs/data/system/migration/upgrade-to-v9/upgrade-to-v9.md#L23-L45, which says those deprecated system props were removed in favor of sx. After this lands, the API pages and Box docs will be corrected, but the getting-started guide will still send users to a removed API.

2. Low: skills/material-ui-styling/AGENTS.md#L120-L127 still instructs agents that Box, Stack, Typography, and Grid accept direct system layout props. Since this repo ships agent guidance for automated edits, leaving that file stale means future generated examples and refactors can keep reintroducing the obsolete behavior this PR is trying to scrub.

Assumptions
I’m assuming master is documenting v9 semantics already; that matches the migration guides checked into the repo. I didn’t find any remaining runtime consumers of cssComponent in the code paths touched by this PR, so the builder/API/LLM removal itself looks internally consistent.

[mj12albert]

A couple Codex suggestions

Updated ~ Box, Stack, Typography, and Grid no longer need to be specifically called out anymore