Revert "Makefile: Prioritize external dtb if defined"

This is causing unexpected size growth in the normal case and is likely
to have been mis-applied by myself.

This reverts commit 3eaf6dcd9362b56e3217559401287dd8fa35b5b2.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/Makefile b/Makefile
index 6eb08a5..059978b 100644
--- a/Makefile
+++ b/Makefile
@@ -1071,7 +1071,7 @@
 u-boot-fit-dtb.bin: u-boot-nodtb.bin $(EXT_DTB)
 		$(call if_changed,cat)
 else
-u-boot-fit-dtb.bin: u-boot-nodtb.bin fit-dtb.blob $(FINAL_DTB_CONTAINER)
+u-boot-fit-dtb.bin: u-boot-nodtb.bin $(FINAL_DTB_CONTAINER)
 	$(call if_changed,cat)
 endif