unlink_nhgrp: Remove write-only variable.

Possibly one could assert that ret should always be 0 here (that is,
that there was always an index found in the bitmask). That should be
true since a bitmask index is allocated before the nhgrp is inserted
in the ctl->gr_head list in link_nhgrp.

(cherry picked from commit 371c917b0b450d1944bf0ea18afdfc158af08f2e)