x86: de-assembler-ize asm/desc.h

asm/desc.h is included in three assembly files, but the only macro
it defines, GET_DESC_BASE, is never used. This patch removes the
includes, removes the macro GET_DESC_BASE and the ASSEMBLY guard
from asm/desc.h.

Signed-off-by: Alexander van Heukelum <heukelum@fastmail.fm>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

arch/x86/kernel/head_32.S

@@ -13,7 +13,6 @@
 #include <asm/segment.h>
 #include <asm/page_types.h>
 #include <asm/pgtable_types.h>
-#include <asm/desc.h>
 #include <asm/cache.h>
 #include <asm/thread_info.h>
 #include <asm/asm-offsets.h>