syscalls.h: remove duplicated declarations for sys_pipe2

sys_pipe2 is declared twice in include/linux/syscalls.h.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>