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/expand_unittest.cc

@@ -83,7 +83,6 @@ class ExpandTest : public ::testing::Test {
                 &statistics_,
                 test_sample_rate_hz_,
                 num_channels_) {
-    WebRtcSpl_Init();
     input_file_.set_output_rate_hz(test_sample_rate_hz_);
   }