Initialize signal processing function pointers statically

The last run-time logic for selecting function pointers was removed in
May 2016, here: https://codereview.webrtc.org/1955413003

It would be even better if we could eliminate the function pointers
entirely and just have different implementations that we select at
compile time; I've left a TODO asking for this.

Bug: webrtc:9553
Change-Id: Ica71d71e19759da00967168f6479b7eb8b46c590
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/144053
Reviewed-by: Alessio Bazzica <alessiob@webrtc.org>
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28414}

modules/audio_coding/neteq/neteq_impl.cc

@@ -133,7 +133,6 @@ NetEqImpl::NetEqImpl(const NetEq::Config& config,
   last_output_sample_rate_hz_ = fs;
   output_size_samples_ = static_cast<size_t>(kOutputSizeMs * 8 * fs_mult_);
   decoder_frame_length_ = 3 * output_size_samples_;
-  WebRtcSpl_Init();
   if (create_components) {
     SetSampleRateAndChannels(fs, 1);  // Default is 1 channel.
   }