Revert "kexec/purgatory: Add clean-up for purgatory directory"

Reverts the following commit:

  b0108f9e ("kexec: purgatory: add clean-up for purgatory directory")

... which incorrectly stated that the kexec-purgatory.c and purgatory.ro files
were not removed after 'make mrproper'.

In fact, they are.  You can confirm it after reverting it.

  $ make mrproper
  $ touch arch/x86/purgatory/kexec-purgatory.c
  $ touch arch/x86/purgatory/purgatory.ro
  $ make mrproper
    CLEAN   arch/x86/purgatory
  $ ls arch/x86/purgatory/
  entry64.S  Makefile  purgatory.c  setup-x86_64.S  stack.S  string.c

This is obvious from the build system point of view.

arch/x86/Makefile adds 'arch/x86' to core-y.
Hence 'make clean' descends like this:

  arch/x86/Kbuild
    -> arch/x86/purgatory/Makefile
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/1529401422-28838-2-git-send-email-yamada.masahiro@socionext.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/Makefile

@@ -327,7 +327,6 @@ archclean:
 	$(Q)rm -rf $(objtree)/arch/x86_64
 	$(Q)$(MAKE) $(clean)=$(boot)
 	$(Q)$(MAKE) $(clean)=arch/x86/tools
-	$(Q)$(MAKE) $(clean)=arch/x86/purgatory
 
 define archhelp
   echo  '* bzImage      - Compressed kernel image (arch/x86/boot/bzImage)'