Bug 1652108 - part1 : remove 'page_count'. r=bryce

`page_count` is a general probe for counting how many top leve content documents have been loaded, which is actully equal to `MIXED_CONTENT_UNBLOCK_COUNTER` [1]. Therefore, we can remove expired probe `page_count` and use `MIXED_CONTENT_UNBLOCK_COUNTER` instead if we want to know the total loaded top level content documents.

[1]
https://searchfox.org/mozilla-central/rev/89814940895946b48b4c04c702efd2c676ec8e7e/toolkit/components/telemetry/Histograms.json#10914-10921
https://searchfox.org/mozilla-central/rev/89814940895946b48b4c04c702efd2c676ec8e7e/dom/ipc/WindowGlobalParent.cpp#696

Differential Revision: https://phabricator.services.mozilla.com/D83222

dom/ipc/WindowGlobalParent.cpp

@@ -994,7 +994,6 @@ void WindowGlobalParent::ActorDestroy(ActorDestroyReason aWhy) {
     }
     Accumulate(Telemetry::MIXED_CONTENT_PAGE_LOAD, mixedContentLevel);
 
-    ScalarAdd(Telemetry::ScalarID::MEDIA_PAGE_COUNT, 1);
     if (GetDocTreeHadAudibleMedia()) {
       ScalarAdd(Telemetry::ScalarID::MEDIA_PAGE_HAD_MEDIA_COUNT, 1);
     }

toolkit/components/telemetry/Scalars.yaml

@@ -2119,25 +2119,6 @@ mediarecorder:
       - content
 
 media:
-  page_count:
-    bug_numbers:
-      - 1476456
-      - 1499803
-      - 1570634
-    description: >
-      The number of times a top level document is loaded.
-    expires: "74"
-    kind: uint
-    notification_emails:
-      - cpearce@mozilla.com
-      - alwu@mozilla.com
-      - nohlmeier@mozilla.com
-    release_channel_collection: opt-out
-    products:
-      - 'firefox'
-    record_in_processes:
-      - content
-
   page_had_media_count:
     bug_numbers:
       - 1476456