[rfc] remove hlt_cpus et al sysctls and related code

I think that it is a well known fact that currently we do not have any support for
dynamically offlining processors.  Yet, we have some code that looks like it does
provide that support and even provides a user interface to supposedly do that.

What we don't currently do specifically:
- rebinding interrupts away from an offlined processor
- updating relevant cpu sets and masks
- protecting the above for concurrent access
- moving threads away from an offlined processor
- notifying potentially interested parties
- maybe more...

The code has been in this shape for a long while and I would dare to say that it
never really worked, not in "production ready" sense anyway.
An example of troubles caused by using that code can be found e.g. in the
followups to the following PR:
http://www.freebsd.org/cgi/query-pr.cgi?pr=145385
And also discussed here:
http://thread.gmane.org/gmane.os.freebsd.stable/74462/focus=74510

I think that there already have been a proposal to remove the systcls and the
code.  I would like to re-submit that proposal.
Removing that code would:
1) prevent users from hurting themselves by executing broken code
2) potentially make things easier for largeSMP project

Once we grow correct code for offlining CPUs, then we could re-introduce the
sysctls without any problems.
While the offlining code doesn't seem terribly hard to develop, it's a big piece
of work and requires time and effort.

-- 
Andriy Gapon