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_device/audio_device_buffer.cc

@@ -65,7 +65,6 @@ AudioDeviceBuffer::AudioDeviceBuffer(TaskQueueFactory* task_queue_factory)
   phase_ = 0.0;
   RTC_LOG(WARNING) << "AUDIO_DEVICE_PLAYS_SINUS_TONE is defined!";
 #endif
-  WebRtcSpl_Init();
 }
 
 AudioDeviceBuffer::~AudioDeviceBuffer() {