contrib: dracut: remove getargbool polyfill

It was originally released in dracut 008 in February 2011;
we can probably drop it now

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13291