Revert "Moved BitrateConfig out of Call::Config."

This reverts commit 5897fe27abcbe70f706cc23adc26147e0581f97e.

Reason for revert: Breaking internal builds

Original change's description:
> Moved BitrateConfig out of Call::Config.
> 
> This prepares for a CL extracting the bitrate configuration logic from
> the Call class.
> 
> Also renaming BitrateConfig to BitrateConstraints.
> 
> Bug: webrtc:8415
> Change-Id: I7e472683034c57bdc8093cdf5e78e477d1732480
> Reviewed-on: https://webrtc-review.googlesource.com/54400
> Commit-Queue: Sebastian Jansson <srte@webrtc.org>
> Reviewed-by: Stefan Holmer <stefan@webrtc.org>
> Reviewed-by: Niels Moller <nisse@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#22104}

TBR=nisse@webrtc.org,stefan@webrtc.org,srte@webrtc.org

Change-Id: I598040edba7f1ff8b39d2d9c3c3ceca5627aaa0c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:8415
Reviewed-on: https://webrtc-review.googlesource.com/55740
Reviewed-by: Lu Liu <lliuu@webrtc.org>
Commit-Queue: Lu Liu <lliuu@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22106}

call/bitrate_constraints.cc

@@ -1,18 +0,0 @@
-/*
- *  Copyright (c) 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.
- */
-
-#include "call/bitrate_constraints.h"
-
-namespace webrtc {
-BitrateConstraintsMask::BitrateConstraintsMask() = default;
-BitrateConstraintsMask::~BitrateConstraintsMask() = default;
-BitrateConstraintsMask::BitrateConstraintsMask(const BitrateConstraintsMask&) =
-    default;
-}  // namespace webrtc