rc.d/wpa_supplicant: Remove the sleep to improve boot time

bapt@ had discovered a noticeable boot improvement without the sleep.
Without the sleep does not affect warm or cold boot however a
service netif restart may cause dhclient to spend a few extra seconds
to rerequest the DHCP request.

Reported by:	bapt
Reviewed by:	bapt
MFC after:	2 months
Differential Revision:	https://reviews.freebsd.org/D35457

libexec/rc/rc.d/wpa_supplicant

@@ -22,7 +22,6 @@ fi
 
 wpa_poststart() {
 	ifconfig ${ifn} down
-	sleep 2
 	ifconfig ${ifn} up
 }