ARM: sunxi: Remove Makefile.boot file

Since we've been a multi-platform enabled kernel from the beginning,
Makefile.boot has always been dead code.

Remove it.
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>

arch/arm/mach-sunxi/Makefile.boot

-zreladdr-$(CONFIG_ARCH_SUNXI)	+= 0x40008000