IA64: Remove NULL flush_write_buffers

flush_write_buffers() in dma-mapping-common.h was removed so we
can remove NULL flush_write_buffers() in IA64.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: tony.luck@intel.com
Cc: fenghua.yu@intel.com
Cc: davem@davemloft.net
LKML-Reference: <1249872797-1314-3-git-send-email-fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>