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: https://github.com/openzfs/openzfs/commit/faa8036
Closes #5544