Illumos 5177 - remove dead code from dsl_scan.c

5177 remove dead code from dsl_scan.c
Reviewed by: George Wilson george.wilson@delphix.com
Reviewed by: Christopher Siden christopher.siden@delphix.com

References:
  https://www.illumos.org/issues/5177
  https://reviews.csiden.org/r/104/

Ported by: Turbo Fredriksson <turbo@bayour.com>