[code-infra] Remove redundant markdownlint-cli2 machinery

[Janpot]

Markdown linting across MUI now runs through ESLint via the markdown: true flag passed to createBaseConfig from @mui/internal-code-infra/eslint. mui-public's own eslint.config.mjs already passes markdown: true, so the separate markdownlint-cli2 self-lint and the shared @mui/internal-code-infra/markdownlint export are redundant and can be removed.

Removed:

• Root .markdownlint-cli2.mjs config, the markdownlint script, and the markdownlint-cli2 devDependency in package.json.
• The packages/code-infra/src/markdownlint/ directory and the ./markdownlint entry from packages/code-infra/package.json's exports map.
• The markdownlint command/job from .circleci/orbs/code-infra.yml (both its definition and its reference in run-linters).
• The markdownlint.config setting from .vscode/settings.json.

The ESLint markdown: true flag stays in place as the replacement.

[code-infra-dashboard]

Deploy preview

https://deploy-preview-1511--mui-internal.netlify.app/

Bundle size

Total Size Change: 0B^(0.00%) - Total Gzip Change: 0B^(0.00%)
Files: 63 total (0 added, 0 removed, 0 changed)

Show details for 63 more bundles

@mui/internal-docs-infra/abstractCreateDemo parsed: 0B^(0.00%) gzip: 0B^(0.00%)
@mui/internal-docs-infra/abstractCreateDemoClient parsed: 0B^(0.00%) gzip: 0B^(0.00%)
@mui/internal-docs-infra/abstractCreateStream parsed: 0B^(0.00%) gzip: 0B^(0.00%)
@mui/internal-docs-infra/abstractCreateTypes parsed: 0B^(0.00%) gzip: 0B^(0.00%)
@mui/internal-docs-infra/ChunkProvider parsed: 0B^(0.00%) gzip: 0B^(0.00%)
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Details of bundle changes

Performance

Total duration: 16.88 ms -3.23 ms^(-16.0%) | Renders: 4 ⁽⁺⁰⁾ | Paint: 70.04 ms -12.25 ms^(-14.9%)

Test	Duration	Renders
DataGrid mount with paint timing	2.63 ms ▼-1.04 ms(-28.2%)	1 (+0)

2 tests within noise — details

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