Retire legacy debugging infrastructure

When the SPL was originally written Linux tracepoints were still
in their infancy.  Therefore, an entire debugging subsystem was
added to facilite tracing which served us well for many years.

Now that Linux tracepoints have matured they provide all the
functionality of the previous tracing subsystem.  Rather than
maintain parallel functionality it makes sense to fully adopt
tracepoints.  Therefore, this patch retires the legacy debugging
infrastructure.

See zfsonlinux/zfs#2406 for the tracepoint intergration changes.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>