Remove obsolete flag warning

src/cli.rs

@@ -49,14 +49,6 @@ preset, regardless of the order you write the arguments.")
                 .value_parser(["0", "1", "2", "3", "4", "5", "6", "max"])
                 .hide_possible_values(true),
         )
-        .arg(
-            Arg::new("backup")
-                .help("Back up modified files")
-                .short('b')
-                .long("backup")
-                .hide(true)
-                .action(ArgAction::SetTrue),
-        )
         .arg(
             Arg::new("recursive")
                 .help("Recurse input directories, optimizing all PNG files")

src/main.rs

@@ -43,11 +43,6 @@ fn main() -> ExitCode {
         .after_long_help("")
         .get_matches_from(std::env::args());
 
-    if matches.get_flag("backup") {
-        error!("The --backup flag is no longer supported. Please use --out or --dir to preserve your existing files.");
-        return ExitCode::FAILURE;
-    }
-
     let (out_file, out_dir, opts) = match parse_opts_into_struct(&matches) {
         Ok(x) => x,
         Err(x) => {