internal/syscall/unix: remove unused *Trap consts

These are unused since the darwin port switched to libc calls in
CL 148457.

Change-Id: I309bb5b0a52c9069484e7a649d4a652efcb8e160
Reviewed-on: https://go-review.googlesource.com/c/go/+/200866
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/internal/syscall/unix/at_sysnum_darwin.go

@@ -4,9 +4,5 @@
 
 package unix
 
-const unlinkatTrap uintptr = 472
-const openatTrap uintptr = 463
-const fstatatTrap uintptr = 470
-
 const AT_REMOVEDIR = 0x80
 const AT_SYMLINK_NOFOLLOW = 0x0020