8602 remove unused "dp_early_sync_tasks" field from "dsl_pool" structure

illumos/illumos-gate@2bcb5458541cc6e8bf7dc541303da29297b82e8b
https://github.com/illumos/illumos-gate/commit/2bcb5458541cc6e8bf7dc541303da29297b82e8b

https://www.illumos.org/issues/8602

When I landed the fix for 8558, I incorrectly added the "dp_early_sync_tasks"
field to the "dsl_pool" structure. This field is used in DelphixOS, but not in
illumos. It was incorrectly pulled into illumos, so this bug is to remove it
from the structure.

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