build: Switch CLEAN back off by default, againHEADmain

Now that we have the notion of a build epoch, we have sufficient tooling
to handle changes that might need a larger hammer to unbreak the build.

Future changes may extend our cleanup to avoid scenarios like with the
MITKRB5 knob where flipping it will still require a cleanup, but on the
whole most users aren't regularly flipping breaking knobs enough for
this to cause the same level of problems.  Those users are advised to do
a CLEAN build when they flip it until we start recording build options
and doing another clean for important knob switches (see D52011).

When knobs of that sort are newly introduced or switch defaults, we can
explicitly record an epoch for them in the interim until
option-switching is detected and handled properly.

This reverts commit 6dab48b9de6c1bff61b0ce78029c1e3cba20895a.

Reviewed by:	jrtc27, markj
Differential Revision:	https://reviews.freebsd.org/D51923

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