GN: Cleanup no longer needed libvpx config.
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: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#8884}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 6b3ccfc6a6e32edba3786f011dc4096eeaf0c68c

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