x86: intel: edison: 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/intel/edison/Makefile b/board/intel/edison/Makefile
index eed8d65..1eaf7ca 100644
--- a/board/intel/edison/Makefile
+++ b/board/intel/edison/Makefile
@@ -4,5 +4,5 @@
 # SPDX-License-Identifier:     GPL-2.0+
 #
 
-obj-y	+= start.o edison.o
+obj-y	+= edison.o
 obj-$(CONFIG_GENERATE_ACPI_TABLE) += dsdt.o