[Tracking Issue] Remove legacy inspector

## Introduction

We would like to remove the legacy Flutter Inspector from DevTools.

## Context

The redesigned Flutter Inspector was launched in [Flutter 3.27](https://medium.com/flutter/whats-new-in-flutter-3-27-28341129570c), and since Flutter 3.29, it has been enabled by default. Currently over 98% of users are using the new inspector, with less than 2% still on the legacy inspector.

## Motivation

Removing this dead code will significantly reduce technical debt, lower the maintenance burden of the inspector tool, and simplify future feature development.

## Scope

The work here is relatively small, though it will need to be coordinated across multiple Flutter releases so that users are warned that the legacy inspector will be removed before it is deleted.