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/flt_spoe.c b/src/flt_spoe.c
index 2d70aff..75854b7 100644
--- a/src/flt_spoe.c
+++ b/src/flt_spoe.c
@@ -1275,9 +1275,7 @@
 	}
 
 	/* Destroy the task attached to this applet */
-	if (spoe_appctx->task) {
-		task_destroy(spoe_appctx->task);
-	}
+	task_destroy(spoe_appctx->task);
 
 	/* Notify all waiting streams */
 	list_for_each_entry_safe(ctx, back, &spoe_appctx->waiting_queue, list) {