mac_bsdextended: Remove \n from sysctl descriptions

sysctl(8) prints a newline after the description; the description should
not end with one itself.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 6b62e00da4ad0624fce5e6f0b5b39a6f44c8ba60)
(cherry picked from commit 28092a8bc72e969d3736ff064ad3f1ca7d80ce0b)