Remove documentation about the legacy inspector from the Flutter website

[elliette]

This should coincide with the release where the legacy inspector code in DevTools is removed.

[aarushlohit]

Can i work IN this or Already fixed Or Anyone Currently Working?? #Newbie

[crackedhandle]

I've opened a PR to fix this issue

Changes made:

• Deleted src/content/tools/devtools/legacy-inspector.md
• Removed the "Use legacy inspector" settings section from inspector.md
• Removed the legacy inspector entry from src/data/sidenav/default.yml
• Updated heading from "The new Flutter inspector" → "The Flutter inspector" and removed the mention of disabling to switch to legacy

[elliette]

This issue should not be addressed until after the legacy inspector is removed from DevTools. Please refer to our contributing guidelines to find good issues for first-time contributors: https://github.com/flutter/devtools/blob/master/CONTRIBUTING.md#welcome