Remove old sigslot.h header

To be landed after 22th Aug 2018

Bug: webrtc:8366
Change-Id: Icaf758cbbb31ec843f5ee85d65963271a98fead0
Reviewed-on: https://webrtc-review.googlesource.com/90412
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24397}

rtc_base/BUILD.gn

@@ -804,7 +804,6 @@ rtc_static_library("rtc_base_generic") {
     "rtccertificategenerator.h",
     "signalthread.cc",
     "signalthread.h",
-    "sigslot.h",
     "sigslotrepeater.h",
     "socket.cc",
     "socket.h",

rtc_base/sigslot.h

@@ -1,14 +0,0 @@
-/*
- *  Copyright 2018 The WebRTC Project Authors. All rights reserved.
- *
- *  Use of this source code is governed by a BSD-style license
- *  that can be found in the LICENSE file in the root of the source
- *  tree. An additional intellectual property rights grant can be found
- *  in the file PATENTS.  All contributing project authors may
- *  be found in the AUTHORS file in the root of the source tree.
- */
-
-// This is a temporary header file to move origin header from this location
-// to specified in include. Don't use this header directly, use included one
-// instead.
-#include "rtc_base/third_party/sigslot/sigslot.h"