Remove obsolete unstable --output-format warning for formatter

The warning that `--output-format` for the formatter requires preview
mode is no longer needed since astral-sh#17138 stabilized formatter output
format support.

Closes astral-sh#23267

https://claude.ai/code/session_0151C636aixLhqvzG7BURQ7P

Author: claude

crates/ruff/src/lib.rs

@@ -212,14 +212,8 @@ pub fn run(
 }
 
 fn format(args: FormatCommand, global_options: GlobalConfigArgs) -> Result<ExitStatus> {
-    let cli_output_format_set = args.output_format.is_some();
     let (cli, config_arguments) = args.partition(global_options)?;
     let pyproject_config = resolve::resolve(&config_arguments, cli.stdin_filename.as_deref())?;
-    if cli_output_format_set && !pyproject_config.settings.formatter.preview.is_enabled() {
-        warn_user_once!(
-            "The --output-format flag for the formatter is unstable and requires preview mode to use."
-        );
-    }
     if is_stdin(&cli.files, cli.stdin_filename.as_deref()) {
         commands::format_stdin::format_stdin(&cli, &config_arguments, &pyproject_config)
     } else {

crates/ruff/tests/cli/format.rs

@@ -2399,25 +2399,6 @@ fn cookiecutter_globbing() -> Result<()> {
     Ok(())
 }
 
-#[test]
-fn stable_output_format_warning() -> Result<()> {
-    let test = CliTest::new()?;
-    assert_cmd_snapshot!(
-        test.format_command()
-            .args(["--output-format=full", "-"])
-            .pass_stdin(""),
-        @"
-    success: true
-    exit_code: 0
-    ----- stdout -----
-
-    ----- stderr -----
-    warning: The --output-format flag for the formatter is unstable and requires preview mode to use.
-    ",
-    );
-    Ok(())
-}
-
 #[test]
 fn markdown_formatting_preview_disabled() -> Result<()> {
     let test = CliTest::new()?;