CLEANUP: task: remove unneeded tests before task_destroy()

Since previous commit it's not needed anymore to test a task pointer
before calling task_destory() so let's just remove these tests from
the various callers before they become confusing. The function's
arguments were also documented. The same should probably be done
with tasklet_free() which involves a test in roughly half of the
call places.
diff --git a/src/haproxy.c b/src/haproxy.c
index f709224..8ab0c2e 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -2290,18 +2290,15 @@
 		while (s) {
 			s_next = s->next;
 
-			if (s->check.task)
-				task_destroy(s->check.task);
-			if (s->agent.task)
-				task_destroy(s->agent.task);
+			task_destroy(s->check.task);
+			task_destroy(s->agent.task);
 
 			if (s->check.wait_list.task)
 				tasklet_free(s->check.wait_list.task);
 			if (s->agent.wait_list.task)
 				tasklet_free(s->agent.wait_list.task);
 
-			if (s->warmup)
-				task_destroy(s->warmup);
+			task_destroy(s->warmup);
 
 			free(s->id);
 			free(s->cookie);