Remove KissFFT from third_party/rnnoise

The modified version of KissFFT that was added only for
webrtc/modules/audio_processing/agc2/rnn_vad is not used anymore.

Bug: webrtc:9577
Change-Id: I7c86d8fa2d277dc2c8a726003b8a495fdd3f172c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1617835
Commit-Queue: Ale Bzk <alessiob@chromium.org>
Reviewed-by: Max Morin <maxmorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661435}
6 files changed