unix: remove Flock syscall for aix

AIX doesn't provide a flock() syscall, it was previously emulated in
package syscall by using fcntl. This emulation was removed in CL 152397.
Since unix.Flock wraps syscall.Flock, the build currently fails on aix
without this change.

Change-Id: Ie887619cd64ae009ea43f00c74e450a5b8f998ac
Reviewed-on: https://go-review.googlesource.com/c/153938
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Clément Chigot <clement.chigot@atos.net>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_aix.go

@@ -13,10 +13,7 @@
 
 package unix
 
-import (
-	"syscall"
-	"unsafe"
-)
+import "unsafe"
 
 /*
  * Wrapped
@@ -385,10 +382,6 @@ func IoctlGetTermios(fd int, req uint) (*Termios, error) {
 
 //sys	fcntl(fd int, cmd int, arg int) (val int, err error)
 
-func Flock(fd int, how int) (err error) {
-	return syscall.Flock(fd, how)
-}
-
 /*
  * Direct access
  */