do_pipe cleanup: drop its last user in arch/alpha/

The last user of do_pipe is in arch/alpha/, after replacing it with
do_pipe_flags, the do_pipe can be totally dropped.

Signed-off-by: Cheng Renquan <crquan@gmail.com>
Acked-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

arch/alpha/kernel/osf_sys.c

@@ -46,8 +46,6 @@
 #include <asm/hwrpb.h>
 #include <asm/processor.h>
 
-extern int do_pipe(int *);
-
 /*
  * Brk needs to return an error.  Still support Linux's brk(0) query idiom,
  * which OSF programs just shouldn't be doing.  We're still not quite