bootdev: drop unnecessary assert on bootflow->bdev

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

Fixes: eccb25cd5922 ("bootstd: Allow the bootdev to be optional in bootflows")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
diff --git a/boot/bootdev-uclass.c b/boot/bootdev-uclass.c
index 35afb93..0fa6dad 100644
--- a/boot/bootdev-uclass.c
+++ b/boot/bootdev-uclass.c
@@ -39,7 +39,6 @@
 	struct bootflow *new;
 	int ret;
 
-	assert(bflow->dev);
 	ret = bootstd_get_priv(&std);
 	if (ret)
 		return ret;