Remove rknpu2 prebuilt paths from library search because they contain
broken symlinks when cloned from Git (symbol link targets not included).
Now CMake searches libraries in priority:
1. User-provided path (RK_MPP_LIB_PATH)
2. Self-built in third_party/mpp/build
3. System default paths (/usr/lib/aarch64-linux-gnu/)
This allows fresh git clone to compile without manual fixes on new RK3588 boards.
Verified: Compiles successfully using build.sh
