x86: efi: efi-x86_payload: Remove dead code

start.S does nothing and can be safely removed. Makefile is still being used
by the build system, so simply drop the rule from it.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/board/efi/efi-x86_payload/Makefile b/board/efi/efi-x86_payload/Makefile
index 00ef695..a4ebc85 100644
--- a/board/efi/efi-x86_payload/Makefile
+++ b/board/efi/efi-x86_payload/Makefile
@@ -2,4 +2,4 @@
 #
 # Copyright (C) 2018, Bin Meng <bmeng.cn@gmail.com>
 
-obj-y	+= start.o payload.o
+obj-y	+= payload.o