Re-enable set but not used warnings for kernel builds.

make tinderbox now passes with this warning enabled as a fatal error,
so revert the change to hide it in preparation for making it fatal.

This reverts commit e8e691983bb75e80153b802f47733f1531615fa2.

Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D34948