Remove debug println

rust/coffi/src/main.rs

    println!("cos(3.1415) = {}", x);
    let mut img = png_image::new();
    println!("{}", img);
    let file_name = std::env::args().nth(1).unwrap_or(String::from("mei.png"));
    let c_name = std::ffi::CString::new(file_name).unwrap();
    let res = img.begin_read_from_file(c_name.as_ptr());