Remove d_thread_t for 8.0

[M. Warner Losh]

In message: <200905121020.18497.jhb at freebsd.org>
            John Baldwin <jhb at freebsd.org> writes:
: In the same vein as purging BURN_BRIDGES stuff, is there any objection to 
: removing d_thread_t from 8.0?  It is intended as a compat shim to reduce 
: diffs with 4.x.  However, at this point drivers are not actively being merged 
: back to 4.x, so I think it is no longer necessary.

It was also intended to allow easier sharing for folks that were using
FreeBSD 4.x, 5.x, etc.  I know that at least one user still has some
4.x deployments, but I suspect that they are otherwise off 4.x so it
might not be a problem for them.  It would be yet another thing to
change when going from 7.x to 8.x for them...

We certainly should remove it from the drivers in the tree for 8.0.
Right now it is used in about a two dozen places.

Warner