nested if statements in command.rs could be made more concise

[hunterchen7]

These nested if statements from src-tauri/src/command.rs can be made more concise:

This one from delete_library_folder():

if folder_path_buf.exists() {
    if folder_path_buf.is_dir() {
        match fs::remove_dir_all(&folder_path_buf) {
            Ok(_) => {},
            Err(e) => eprintln!("Error removing folder {}: {:?}", folder_id, e),
        }
    }
}

can be replaced with:

if folder_path_buf.exists() && folder_path_buf.is_dir() {
    match fs::remove_dir_all(&folder_path_buf) {
        Ok(_) => {},
        Err(e) => eprintln!("Error removing folder {}: {:?}", folder_id, e),
    }
}

and this one from delete_library_paper():

if paper_filepath_buf.exists() {
    if paper_filepath_buf.is_file() {
        match fs::remove_file(&paper_filepath_buf) {
            Ok(_) => {},
            Err(e) => eprintln!("Error removing folder {}: {:?}", folder_id, e),
        }
    }
}

with:

if paper_filepath_buf.exists() && paper_filepath_buf.is_file() {
    match fs::remove_file(&paper_filepath_buf) {
        Ok(_) => {},
        Err(e) => eprintln!("Error removing folder {}: {:?}", folder_id, e),
    }
}

[hunterchen7]

resolved with PR #70