Revert of RTCInboundRTPStreamStats.qpSum collected. (patchset #4 id:80001 of https://codereview.webrtc.org/2675943002/ )

Reason for revert:
Breaks downstream build.

Original issue's description:
> RTCInboundRTPStreamStats.qpSum collected.
>
> This was previously only collected for local tracks
> (RTCOutboundRTPStreamStats.qpSum).
>
> Spec: https://w3c.github.io/webrtc-stats/#dom-rtcrtpstreamstats-qpsum
>
> This CL also improves some testing in rtcstatscollector_unittest.cc.
> Default and non-default values are tested in the same unittests,
> removing the test that was specific to default-values, which was
> otherwise code duplication.
>
> BUG=webrtc:7065
>
> Review-Url: https://codereview.webrtc.org/2675943002
> Cr-Commit-Position: refs/heads/master@{#16477}
> Committed: cd195bea5e

TBR=sakal@webrtc.org,hta@webrtc.org,hbos@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:7065

Review-Url: https://codereview.webrtc.org/2687483002 .
Cr-Commit-Position: refs/heads/master@{#16479}

webrtc/api/stats/rtcstats_objects.h

@@ -323,6 +323,8 @@ class RTCRTPStreamStats : public RTCStats {
   // TODO(hbos): Not collected by |RTCStatsCollector|. crbug.com/657854
   // SLI count is only defined for |media_type == "video"|.
   RTCStatsMember<uint32_t> sli_count;
+  // TODO(hbos): Only collected for the outbound case, should also be collected
+  // for inbound case by |RTCStatsCollector|. crbug.com/657854, crbug.com/657855
   RTCStatsMember<uint64_t> qp_sum;
 
  protected: