avr32: fix nop compile fails from system.h split up

To fix:

  In file included from kernel/exit.c:61:
  arch/avr32/include/asm/mmu_context.h: In function 'enable_mmu':
  arch/avr32/include/asm/mmu_context.h:135: error: implicit declaration of function 'nop'

It needs an include of the new file created in commit ae473946
("Disintegrate asm/system.h for AVR32"), but since that file only
contains "nop", and since other arch already have precedent of putting
nop in asm/barrier.h we should just delete the new file and put nop in
barrier.h

Suggested-and-acked-by: David Howells <dhowells@redhat.com>
Cc: Haavard Skinnemoen <hskinnemoen@gmail.com>
Cc: Hans-Christian Egtvedt <egtvedt@samfundet.no>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>