syscall: remove un-used const ptrSize

Change-Id: Ic809a533f9c4042373bdad3ba1cd237d203bacff
Reviewed-on: https://go-review.googlesource.com/c/go/+/202881Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/syscall/syscall_darwin.go

@@ -380,8 +380,6 @@ func writelen(fd int, buf *byte, nbuf int) (n int, err error) {
 
 func Getdirentries(fd int, buf []byte, basep *uintptr) (n int, err error) {
 	// Simulate Getdirentries using fdopendir/readdir_r/closedir.
-	const ptrSize = unsafe.Sizeof(uintptr(0))
-
 	// We store the number of entries to skip in the seek
 	// offset of fd. See issue #31368.
 	// It's not the full required semantics, but should handle the case