Clean up legacy rn_inspector entry point

[huntie]

Summary

Note

Depends on facebook/react-native#55548 landing in the same React Native release.

Removes the legacy rn_inspector.html entry point, which is no longer in use.

Changes (Claude Code summary)

Deleted:

• front_end/entrypoints/rn_inspector/ directory (3 files: rn_inspector.ts, BUILD.gn, README.md)
• front_end/panels/rn_welcome/rn_welcome-legacy-meta.ts

Updated (planned changes):

• front_end/panels/rn_welcome/BUILD.gn — removed meta-legacy entrypoint target
• front_end/panels/rn_welcome/RNWelcome.ts — removed showBetaLabel, showTechPreviewLabel options and their UIStrings
  (betaLabel, techPreviewLabel), plus the conditional badge rendering blocks
• front_end/panels/rn_welcome/rnWelcome.css — removed .rn-welcome-title-accessory and
  .rn-welcome-title-accessory-purple rules
• front_end/BUILD.gn — removed rn_inspector entrypoint dep and template entry
• config/gni/devtools_grd_files.gni — removed 3 rn_inspector/legacy-meta GRD entries
• scripts/build/generate_html_entrypoint.js — removed rn_inspector from title map
• front_end/core/host/RNPerfMetrics.ts — changed default #entryPoint to 'rn_fusebox', simplified EntryPoint type

Updated (additional cleanup found via grep):

• front_end/entrypoints/main/BUILD.gn — removed rn_inspector visibility
• front_end/entrypoints/inspector_main/BUILD.gn — removed rn_inspector visibility
• front_end/entrypoints/shell/BUILD.gn — removed rn_inspector visibility
• front_end/core/rn_experiments/BUILD.gn — removed rn_inspector visibility
• front_end/panels/recorder/BUILD.gn — removed rn_inspector visibility
• front_end/core/rn_experiments/experimentsImpl.ts — updated comment
• front_end/index.html — removed rn_inspector.html link
• front_end/panels/rn_welcome/rn_welcome-meta.ts — removed dead showBetaLabel: false
• scripts/eslint_rules/lib/check-license-header.js — removed rn_inspector entry
• README.md — updated URL reference from rn_inspector to rn_fusebox

Final grep confirms zero remaining references to rn_inspector, rn_welcome-legacy-meta, showBetaLabel,
showTechPreviewLabel, or rn-welcome-title-accessory.

Test plan

npm run build

Upstreaming plan

• This commit should be sent as a patch to the upstream devtools-frontend repo following the contribution guide for Meta employees OR contribution guide.
• This commit is React Native-specific and cannot be upstreamed.