7259 DS_FIELD_LARGE_BLOCKS is unused

The DS_FIELD_LARGE_BLOCKS macro has been unused since the integration of
this patch:

commit ca0cc3918a1789fa839194af2a9245f801a06b1a
Author: Matthew Ahrens <mahrens@delphix.com>
Date:   Fri Jul 24 09:53:55 2015 -0700

    5959 clean up per-dataset feature count code
    Reviewed by: Toomas Soome <tsoome@me.com>
    Reviewed by: George Wilson <george@delphix.com>
    Reviewed by: Alex Reece <alex@delphix.com>
    Approved by: Richard Lowe <richlowe@richlowe.net>

This patch simply removes this macro from dsl_dataset.h.

Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Reviewed by: Igor Kozhukhov <ikozhukhov@gmail.com>
Author: Matthew Ahrens <mahrens@delphix.com>