x86/boot: Remove unused 'is_big_kernel' variable

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:

  5e47c478b0b6 ("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.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>