build: Switch CLEAN back on by default

There have been too many issues with non-META_MODE incremental builds
recently, and it is clear that most users, even developers, were not
aware that the default had been switched.

This will be revisited once more work has been done to help prevent
future breakage.

This reverts commit ba373fca78a114768244d6a8c27983da870c1169.

Reviewed by: markj, jhb
Differential Revision: https://reviews.freebsd.org/D51828