[Infra:Chore] Remove NeuroPilot implementation from public repository

[wangzhaode]

Summary

• Remove the NeuroPilot backend implementation from the public repository.
• Keep a public README explaining the authorization requirement and support contacts.
• Replace the backend CMake configuration with a placeholder that warns users and disables MNN_NEUROPILOT.

Motivation

The NeuroPilot implementation is not distributed as part of the open-source repository and requires authorization from MediaTek.

Impact

Default builds are unaffected. If a user explicitly enables MNN_NEUROPILOT, CMake emits a non-fatal warning, disables the option, and continues configuration.

Validation

• Configured with -DMNN_NEUROPILOT=ON and verified that CMake completed successfully.
• Verified MNN_NEUROPILOT:BOOL=OFF in the generated CMake cache.
• Passed changed-line clang-format, large-file, and diff checks.