kqueue: Fix kqueue(2) man page.

Remove bogus BUGS note about timeout limit to 24 hours, that's not true
since callouting project import.

Reviewed by: mav
Differential revision: https://reviews.freebsd.org/D35206
MFC after: 2 weeks

(cherry picked from commit 14c99b43ef69beafa01a1574641c4cf19ecf2b9d)