dm: mmc: remove unused U_BOOT_DRIVER(mmc)

When look through the code, I found this bare metal
drives is not used, so remove it.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/mmc/mmc-uclass.c b/drivers/mmc/mmc-uclass.c
index 2b146ea..4a12324 100644
--- a/drivers/mmc/mmc-uclass.c
+++ b/drivers/mmc/mmc-uclass.c
@@ -427,10 +427,6 @@
 };
 #endif /* CONFIG_BLK */
 
-U_BOOT_DRIVER(mmc) = {
-	.name	= "mmc",
-	.id	= UCLASS_MMC,
-};
 
 UCLASS_DRIVER(mmc) = {
 	.id		= UCLASS_MMC,