kernel: Remove numerous #include <sys/thread2.h>.

Most of them were added when we converted spl*() calls to
crit_enter()/crit_exit(), almost 14 years ago. We can now
remove a good chunk of them agai

kernel: Remove numerous #include <sys/thread2.h>.

Most of them were added when we converted spl*() calls to
crit_enter()/crit_exit(), almost 14 years ago. We can now
remove a good chunk of them again for where crit_*() are
no longer used.

I had to adjust some files that were relying on thread2.h
or headers that it includes coming in via other headers
that it was removed from.

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