Cleanup: Delete dead code from send_merge_thread()

range is always deferenced before it reaches this check, such that the
kmem_zalloc() call is never executed.

There is also no need to set `range->eos_marker = B_TRUE` because it is
already set.

Coverity incorrectly complained about a potential NULL pointer
dereference because of this.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes openzfs#14042

module/zfs/dmu_send.c

@@ -1586,9 +1586,8 @@ send_merge_thread(void *arg)
 		}
 		range_free(front_ranges[i]);
 	}
-	if (range == NULL)
-		range = kmem_zalloc(sizeof (*range), KM_SLEEP);
-	range->eos_marker = B_TRUE;
+	ASSERT3P(range, !=, NULL);
+	ASSERT3S(range->eos_marker, ==, B_TRUE);
 	bqueue_enqueue_flush(&smt_arg->q, range, 1);
 	spl_fstrans_unmark(cookie);
 	thread_exit();