bootdev: drop unnecessary assert on bootflow->bdev

Not all flows have a device and the function already contains logic to
handle this case.

Fixes: eccb25cd59 ("bootstd: Allow the bootdev to be optional in bootflows")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>

boot/bootdev-uclass.c

@@ -39,7 +39,6 @@ int bootdev_add_bootflow(struct bootflow *bflow)
 	struct bootflow *new;
 	int ret;
 
-	assert(bflow->dev);
 	ret = bootstd_get_priv(&std);
 	if (ret)
 		return ret;