ACM: Removed the code for InitialDelayManager

It looks to have been unused since the landing of
https://codereview.webrtc.org/1419573013

BUG=webrtc:3520

Review-Url: https://codereview.webrtc.org/2363993002
Cr-Commit-Position: refs/heads/master@{#14397}

webrtc/modules/audio_coding/BUILD.gn

@@ -92,8 +92,6 @@ rtc_static_library("audio_coding") {
     "acm2/call_statistics.h",
     "acm2/codec_manager.cc",
     "acm2/codec_manager.h",
-    "acm2/initial_delay_manager.cc",
-    "acm2/initial_delay_manager.h",
     "include/audio_coding_module.h",
     "include/audio_coding_module_typedefs.h",
   ]