Remove Emscripten support

lib/Target/JSBackend/TargetInfo/JSBackendTargetInfo.cpp

-//===-- JSBackendTargetInfo.cpp - JSBackend Target Implementation -------===//
-//
-//                     The LLVM Compiler Infrastructure
-//
-// This file is distributed under the University of Illinois Open Source
-// License. See LICENSE.TXT for details.
-//
-//===--------------------------------------------------------------------===//
-
-#include "JSTargetMachine.h"
-#include "MCTargetDesc/JSBackendMCTargetDesc.h"
-#include "llvm/IR/Module.h"
-#include "llvm/Support/TargetRegistry.h"
-using namespace llvm;
-
-Target llvm::TheJSBackendTarget;
-
-extern "C" void LLVMInitializeJSBackendTargetInfo() { 
-  RegisterTarget<Triple::asmjs, /*HasJIT=*/false> X(TheJSBackendTarget, "js", "JavaScript (asm.js, emscripten) backend");
-}