Stabilize cargo clippy --fix

This has been unstable since it was first introduced in
rust-lang#5363. In that time, I have
been using it successfully in nightly without issues. I don't think
there are any blocking issues now that RUSTC_WORKSPACE_WRAPPER is
stabilized, so this can be stabilized.

src/main.rs

@@ -70,7 +70,6 @@ impl ClippyCmd {
         I: Iterator<Item = String>,
     {
         let mut cargo_subcommand = "check";
-        let mut unstable_options = false;
         let mut args = vec![];
 
         for arg in old_args.by_ref() {
@@ -80,18 +79,12 @@ impl ClippyCmd {
                     continue;
                 },
                 "--" => break,
-                // Cover -Zunstable-options and -Z unstable-options
-                s if s.ends_with("unstable-options") => unstable_options = true,
                 _ => {},
             }
 
             args.push(arg);
         }
 
-        if cargo_subcommand == "fix" && !unstable_options {
-            panic!("Usage of `--fix` requires `-Z unstable-options`");
-        }
-
         let mut clippy_args: Vec<String> = old_args.collect();
         if cargo_subcommand == "fix" && !clippy_args.iter().any(|arg| arg == "--no-deps") {
             clippy_args.push("--no-deps".into());