Remove remaining quality-analysis (QM).

This was never turned on, contains a lot of complexity and somehow
manages triggering a bug in a downstream project.

BUG=webrtc:5066
R=marpan@webrtc.org
TBR=mflodman@webrtc.org

Review URL: https://codereview.webrtc.org/1917323002 .

Cr-Commit-Position: refs/heads/master@{#12692}

webrtc/modules/video_coding/BUILD.gn

@@ -14,8 +14,6 @@ source_set("video_coding") {
     "codec_database.h",
     "codec_timer.cc",
     "codec_timer.h",
-    "content_metrics_processing.cc",
-    "content_metrics_processing.h",
     "decoding_state.cc",
     "decoding_state.h",
     "encoded_frame.cc",
@@ -54,9 +52,6 @@ source_set("video_coding") {
     "packet_buffer.h",
     "percentile_filter.cc",
     "percentile_filter.h",
-    "qm_select.cc",
-    "qm_select.h",
-    "qm_select_data.h",
     "receiver.cc",
     "receiver.h",
     "rtt_filter.cc",