Revert "implement std::process::Command::args_new()"

This reverts commit 626d0e71b1e82c4804cec1a2bd03c3c53e4368ad.

* removes 'args_new()' as requested in #87420 (comment)

* still disallow argv0 mutation as I explained in #87420 (comment)

Author: cehteh

library/std/src/process.rs

@@ -717,48 +717,6 @@ impl Command {
         self
     }
 
-    /// Reset the argument list to new content
-    ///
-    /// Allows to maintain the argument list in a Vec or anything else that can provide
-    /// a suitable iterator.
-    ///
-    /// # Examples
-    ///
-    /// Basic usage:
-    ///
-    /// ```no_run
-    /// #![feature(mutate_command_args)]
-    /// use std::process::Command;
-    ///
-    /// // Prepare a command
-    /// let mut command = Command::new("ls");
-    /// let mut my_args = Vec::from(["-l", "foo"]);
-    ///
-    /// command
-    ///     .args_new(&my_args)
-    ///     .spawn()
-    ///     .unwrap();
-    ///
-    /// // Mutate my_args
-    /// my_args.insert(1, "-a");
-    /// my_args.pop();
-    /// my_args.push("bar");
-    ///
-    /// // Run command again with the mutated arguments
-    /// command
-    ///     .args_new(&my_args)
-    ///     .spawn()
-    ///     .unwrap();
-    /// ```
-    #[unstable(feature = "mutate_command_args", issue = "87379")]
-    pub fn args_new<I, S>(&mut self, args: I) -> &mut Command
-    where
-        I: IntoIterator<Item = S>,
-        S: AsRef<OsStr>,
-    {
-        self.args_clear().args(args)
-    }
-
     /// Inserts or updates an environment variable mapping.
     ///
     /// Note that environment variable names are case-insensitive (but case-preserving) on Windows,