LinuxKPI: implement timer_{delete,shutdown}_sync()

Implement timer_{delete,shutdown}_sync(), which do not seem to require
anything additional to the already existing del_timer_sync().

Sponsored by: The FreeBSD Foundation
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D40124

(cherry picked from commit 2e07e885d65973f13d6111f7e0fcfb848a68082e)