bootstd: Drop the system bootdev

This was a work-around for the fact that global bootmeths such as EFI
bootmgr and VBE don't use a particular bootdev, or at least select it
themselves so that we don't need to scan all bootdevs when using that
bootmeth.

Drop the system bootdev entirely.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/Makefile b/boot/Makefile
index 124065a..854b391 100644
--- a/boot/Makefile
+++ b/boot/Makefile
@@ -18,7 +18,7 @@
 obj-$(CONFIG_ANDROID_AB) += android_ab.o
 obj-$(CONFIG_ANDROID_BOOT_IMAGE) += image-android.o image-android-dt.o
 
-obj-$(CONFIG_$(SPL_TPL_)BOOTSTD) += bootdev-uclass.o system_bootdev.o
+obj-$(CONFIG_$(SPL_TPL_)BOOTSTD) += bootdev-uclass.o
 obj-$(CONFIG_$(SPL_TPL_)BOOTSTD) += bootflow.o
 obj-$(CONFIG_$(SPL_TPL_)BOOTSTD) += bootmeth-uclass.o
 obj-$(CONFIG_$(SPL_TPL_)BOOTSTD) += bootstd-uclass.o