x86/build: Remove unnecessary preparation for purgatory

kexec-purgatory.c is properly generated when Kbuild descend into
the arch/x86/purgatory/.

Thus the 'archprepare' target is redundant.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michal Marek <michal.lkml@markovi.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/lkml/1529401422-28838-3-git-send-email-yamada.masahiro@socionext.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>