Stop adding -Wredundant-decls to CWARNFLAGS.

clang doesn't implement it, and Linux doesn't enforce it. As a
result, new instances keep cropping up both in FreeBSD's code and in
upstream sources from vendors.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D34144

(cherry picked from commit c227269e2f9dc2695d00be9e5a72b5fdeec5712a)