MFV r323914: 8661 remove "zil-cw2" dtrace probe

illumos/illumos-gate@bd9d3f904625846bdc61af8897a1072029c7aeb7
https://github.com/illumos/illumos-gate/commit/bd9d3f904625846bdc61af8897a1072029c7aeb7

https://www.illumos.org/issues/8661

The "zil-cw1" dtrace probe was previously removed in 8558, and the "zil-cw2"
probe should have been removed in that patch as well. Unfortunately, the "zil-
cw2" was not removed in 8558, so this bug is to track it's removal.

Reviewed by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Igor Kozhukhov <igor@dilos.org>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Prakash Surya <prakash.surya@delphix.com>

MFC after: 1 week