OpenZFS 7259 - DS_FIELD_LARGE_BLOCKS is unused

Authored by: Matthew Ahrens <mahrens@delphix.com>
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>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>

The DS_FIELD_LARGE_BLOCKS macro has been unused since the integration of
this patch: 241b541 Illumos 5959 - clean up per-dataset feature count code.

This patch simply removes this macro from dsl_dataset.h.

OpenZFS-issue: https://www.illumos.org/issues/7259
OpenZFS-commit: openzfs/openzfs@faa8036
Closes #5544

include/sys/dsl_dataset.h

@@ -84,13 +84,6 @@ struct dsl_pool;
  */
 #define	DS_FIELD_BOOKMARK_NAMES "com.delphix:bookmarks"
 
-/*
- * This field is present (with value=0) if this dataset may contain large
- * blocks (>128KB).  If it is present, then this dataset
- * is counted in the refcount of the SPA_FEATURE_LARGE_BLOCKS feature.
- */
-#define	DS_FIELD_LARGE_BLOCKS "org.open-zfs:large_blocks"
-
 /*
  * This field is present (with value=0) if this dataset may contain large
  * dnodes (>512B).  If it is present, then this dataset is counted in the