Remove sdt.h

It's mostly a noop on ZoL and it conflicts with platforms that
support dtrace. Remove this header to resolve the conflict.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9497