Disable REPRODUCIBLE_BUILD for kernel builds.

The REPRODUCIBLE_BUILD option is actually managed in two separate
files. src.opts.mk governs the setting for world builds and
kern.opts.mk governs it for kernel builds. r350550 only changed the
default for world builds.

Reported by: emaste
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D21444