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: https://github.com/nodejs/node/pull/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>