Remove SampleLegacyModule from RNTester (#55991)

Summary:

The `SampleLegacyModule` was a sample module used to demonstrate the legacy (non-TurboModule) native module API. Since React Native has fully migrated to TurboModules, this legacy sample module is no longer needed.

This removes:
- `SampleLegacyModule.kt` (Android platform implementation)
- `RCTSampleLegacyModule.h/mm` (iOS platform implementation)
- `LegacyModuleExample.js` and `SampleLegacyModuleExample.js` (JS example screens)
- All references from `RNTesterApplication.kt`, `CatalystApplication.java`, and RNTester list files
- E2E test for SampleLegacyModule

## Changelog:

[General] [Removed] - Remove SampleLegacyModule from RNTester

Differential Revision: D95684553

Author: christophpurrer