Remove deprecated legacy AEC code

This CL removes the deprecated legacy AEC code.

Note that this CL should not be landed before the M80 release has been cut.

Bug: webrtc:11165
Change-Id: I59ee94526e62f702bb9fa9fa2d38c4e48f44753c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/161238
Commit-Queue: Per Åhgren <peah@webrtc.org>
Reviewed-by: Gustaf Ullberg <gustaf@webrtc.org>
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30036}

modules/audio_processing/audio_processing_unittest.cc

@@ -1536,8 +1536,6 @@ TEST_F(ApmTest, Process) {
 
     Config config;
     config.Set<ExperimentalAgc>(new ExperimentalAgc(false));
-    config.Set<ExtendedFilter>(
-        new ExtendedFilter(test->use_aec_extended_filter()));
     apm_.reset(AudioProcessingBuilder().Create(config));
 
     EnableAllComponents();