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_processing/aecm/echo_control_mobile.cc

@@ -85,8 +85,6 @@ static int WebRtcAecm_DelayComp(AecMobile* aecm);
 void* WebRtcAecm_Create() {
   AecMobile* aecm = static_cast<AecMobile*>(malloc(sizeof(AecMobile)));
 
-  WebRtcSpl_Init();
-
   aecm->aecmCore = WebRtcAecm_CreateCore();
   if (!aecm->aecmCore) {
     WebRtcAecm_Free(aecm);

modules/audio_processing/ns/noise_suppression_x.c

@@ -18,7 +18,6 @@
 
 NsxHandle* WebRtcNsx_Create() {
   NoiseSuppressionFixedC* self = malloc(sizeof(NoiseSuppressionFixedC));
-  WebRtcSpl_Init();
   self->real_fft = NULL;
   self->initFlag = 0;
   return (NsxHandle*)self;