dm: usb: Drop the get_dev() function

This function is implemented by the legacy block functions now. Drop it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/disk/part.c b/disk/part.c
index f055c74..1b33928 100644
--- a/disk/part.c
+++ b/disk/part.c
@@ -32,7 +32,7 @@
 	{ .name = "scsi", .get_dev = scsi_get_dev, },
 #endif
 #if defined(CONFIG_CMD_USB) && defined(CONFIG_USB_STORAGE)
-	{ .name = "usb", .get_dev = usb_stor_get_dev, },
+	{ .name = "usb", },
 #endif
 #if defined(CONFIG_MMC)
 	{