Delete deprecated AEC interfaces

They've been officially deprecated since September 4, 2018.
PSA: https://groups.google.com/forum/#!topic/discuss-webrtc/r_9n-PRUIX4

Bug: webrtc:9535
Change-Id: I294e22ae874b1edd81a0a0347755d82c5ebc61e0
Reviewed-on: https://webrtc-review.googlesource.com/c/103444
Reviewed-by: Gustaf Ullberg <gustaf@webrtc.org>
Commit-Queue: Sam Zackrisson <saza@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24971}
diff --git a/modules/audio_processing/echo_control_mobile_impl.h b/modules/audio_processing/echo_control_mobile_impl.h
index 3341ec5..a5b66c8 100644
--- a/modules/audio_processing/echo_control_mobile_impl.h
+++ b/modules/audio_processing/echo_control_mobile_impl.h
@@ -24,6 +24,8 @@
 
 class AudioBuffer;
 
+// The acoustic echo control for mobile (AECM) component is a low complexity
+// robust option intended for use on mobile devices.
 class EchoControlMobileImpl {
  public:
   EchoControlMobileImpl(rtc::CriticalSection* crit_render,