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