runtime/cgo: remove TMPDIR logic for iOS

Seems like the simplest solution for 1.5. All the parts of the test
suite I can run on my current device (for which my exception handler
fix no longer works, apparently) pass without this code. I'll move it
into x/mobile/app.

Fixes #11884

Change-Id: I2da40c8c7b48a4c6970c4d709dd7c148a22e8727
Reviewed-on: https://go-review.googlesource.com/12721Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/cgo/tmpdir_darwin.go

-// Copyright 2015 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// +build arm arm64
-
-package cgo
-
-/*
-#cgo CFLAGS: -x objective-c
-#cgo LDFLAGS: -framework CoreFoundation -framework Foundation
-
-#include <sys/param.h>
-#include <CoreFoundation/CFString.h>
-#include <Foundation/NSPathUtilities.h>
-
-char tmpdir[MAXPATHLEN];
-
-char* loadtmpdir() {
-	tmpdir[0] = 0;
-	CFStringRef path = (CFStringRef)NSTemporaryDirectory();
-	CFStringGetCString(path, tmpdir, sizeof(tmpdir), kCFStringEncodingUTF8);
-	return tmpdir;
-}
-*/
-import "C"
-
-func init() {
-	if Getenv("TMPDIR") != "" {
-		return
-	}
-	dir := C.GoString(C.loadtmpdir())
-	if len(dir) == 0 {
-		return
-	}
-	if dir[len(dir)-1] == '/' {
-		dir = dir[:len(dir)-1]
-	}
-	Setenv("TMPDIR", dir)
-}