Remove RelatedHistogramSet.

docs/histogram-set-json-format.md

 <!-- Copyright 2016 The Chromium Authors. All rights reserved.
      Use of this source code is governed by a BSD-style license that can be
      found in the LICENSE file.
 -->
 
 # HistogramSet JSON Format
 
 This document assumes familiarity with the concepts introduced in
 [how-to-write-metrics](/docs/how-to-write-metrics.md).
 
@@ skipping 111 matching lines @@
    specify which summary statistics are interesting and should be displayed.
 
 DiagnosticMap is a dictionary mapping strings to Diagnostic dictionaries.
 
 ## Diagnostics
 
 The only field that is required for all Diagnostics, `type`, must be one of
  * `GenericSet`
  * `RelatedEventSet`
  * `Breakdown`
- * `RelatedHistogramSet`
  * `RelatedHistogramMap`
  * `RelatedHistogramBreakdown`
  * `TelemetryInfo`
  * `BuildbotInfo`
  * `Scalar`
 
 If a Diagnostic is in the root array of the JSON, then it is shared, so it may be
 referenced by multiple Histograms. Shared Diagnostics must contain a string
 field `guid` containing a UUID.
 
@@ skipping 40 matching lines @@
  * `value`: a dictionary containing a string `unit` and a number `value`
 
 ### Breakdown
 
 This allows metrics to explain the magnitude of a sample as composed of various
 categories.
 
  * `values`: required dictionary mapping from a string category name to number values
  * `colorScheme`: optional string specifying how the bar chart should be colored
 
-### RelatedHistogramSet
-
-This allows metrics to annotate which Histograms are related to other
-Histograms.
-
- * `guids`: list of guids of related Histograms
-
 ### RelatedHistogramMap
 
 This allows metrics to annotate which Histograms are related to other
 Histograms, and annotate the nature of the relationship with a custom string
 name.
 
  * `values`: dictionary mapping from custom string name to the related
    Histogram's string guid
 
 ### RelatedHistogramBreakdown
@@ skipping 11 matching lines @@
 
  * `events`: array of dictionaries containing `stableId`, `title`, `start`,
    `duration` fields of Events
 
 ### DateRange
 
 This is a Range of Dates.
 
  * `min`: Unix timestamp in ms
  * `max`: optional Unix timestamp in ms