Remove OS-specific build instructions from README

Consolidated build instructions by removing separate Linux/macOS and Windows sections. This simplifies the README and avoids duplication.

Author: lijy91

README.md

@@ -48,8 +48,6 @@ git clone https://github.com/libnativeapi/nativeapi.git
 cd nativeapi
 ```
 
-#### Linux/macOS
-
 ```bash
 # Create build directory
 mkdir build
@@ -60,18 +58,6 @@ cmake ..
 cmake --build . --config Release
 ```
 
-#### Windows
-
-```powershell
-# Create build directory
-mkdir build
-cd build
-
-# Configure and build
-cmake ..
-cmake --build . --config Release
-```
-
 ## Development
 
 ### Code Formatting