Revert "MIPS: boot: Define __ASSEMBLY__ for its.S build"

This reverts commit 0f9da844.

It is true that commit 0f9da844 ("MIPS: boot: Define __ASSEMBLY__
for its.S build") fixed the build error, but it should not have
defined __ASSEMBLY__ just for textual substitution in arbitrary data.
The file is image tree source in this case, but the purpose of using
CPP is to replace some macros.

I merged a better solution, commit a95b37e2

 ("kbuild: get
<linux/compiler_types.h> out of <linux/kconfig.h>").  The original
fix-up is no longer needed.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Patchwork: https://patchwork.linux-mips.org/patch/19096/
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: linux-kernel@vger.kernel.org