Remove audio_mixer_manager_win.cc/.h.

Not used after Wave support dropped in https://codereview.webrtc.org/2700983002/.

BUG=webrtc:7183

Review-Url: https://codereview.webrtc.org/2699333002
Cr-Original-Commit-Position: refs/heads/master@{#16690}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: e654b6387929d0b2cef5f8b2d1c9fa14b23f9cba
3 files changed