x86/boot: Remove unused 'is_big_kernel' variable

Variable is_big_kernel is defined in arch/x86/boot/tools/build.c but
never used anywhere.

Boris noted that its usage went away 7 years ago, as of:

  5e47c478 ("x86: remove zImage support")
Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1455453358-4088-1-git-send-email-nicolas.iooss_linux@m4x.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/boot/tools/build.c

@@ -49,7 +49,6 @@ typedef unsigned int   u32;
 
 /* This must be large enough to hold the entire setup */
 u8 buf[SETUP_SECT_MAX*512];
-int is_big_kernel;
 
 #define PECOFF_RELOC_RESERVE 0x20