MFC r305563: MFV r305562: 7259 DS_FIELD_LARGE_BLOCKS is unused

MFC r305563: MFV r305562: 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>

git-svn-id: svn://svn.freebsd.org/base/stable/10@307300 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f