GN: Cleanup no longer needed libvpx config.

The includes this config provided are now
present just by depending on libvpx.

R=tfarina@chromium.org

Review URL: https://webrtc-codereview.appspot.com/44949004

Cr-Original-Commit-Position: refs/heads/master@{#8884}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 6b3ccfc6a6e32edba3786f011dc4096eeaf0c68c
1 file changed