lib: remove unused exec param

`exec`'s parameters are now deconstructored through
`normalizeExecArgs.apply`. We don't need an exclipit parameter anymore
(and in fact it's NEVER referred in the code directly), like `spwan` or
`spwanSync`. This might be missing.

PR-URL: #22274
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: George Adams <george.adams@uk.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Jon Moss <me@jonathanmoss.me>

lib/child_process.js

@@ -142,8 +142,8 @@ function normalizeExecArgs(command, options, callback) {
 }
 
 
-exports.exec = function exec(command /* , options, callback */) {
-  var opts = normalizeExecArgs.apply(null, arguments);
+exports.exec = function exec(/* command , options, callback */) {
+  const opts = normalizeExecArgs.apply(null, arguments);
   return exports.execFile(opts.file,
                           opts.options,
                           opts.callback);