unix: remove un-used ptrSize

Same as CL 202881.

Change-Id: Idc85bd23574c437c711b40155b49621e35ef571b
Reviewed-on: https://go-review.googlesource.com/c/sys/+/203237
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/syscall_darwin.1_13.go

diff --git a/unix/syscall_darwin.1_13.go b/unix/syscall_darwin.1_13.go
index 24960c3..f911617 100644
--- a/unix/syscall_darwin.1_13.go
+++ b/unix/syscall_darwin.1_13.go
@@ -27,8 +27,6 @@
 
 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