Bug 1421547 - Remove remaining traces of the old toolkit.telemetry.enabledPreRelease pref. r=rnewman

The code to migrate from the toolkit.telemetry.enabledPreRelease pref to
toolkit.telemetry.enabled was added to Firefox 31 in bug 986582. It should be
safe to remove now.

MozReview-Commit-ID: JBkn20bUQXx

--HG--
extra : rebase_source : 1fa65f1f5b8b6251af7a888959d931652363fc9a

modules/libpref/Preferences.cpp

@@ -2829,7 +2829,6 @@ static nsresult
 pref_LoadPrefsInDirList(const char* aListId);
 
 static const char kTelemetryPref[] = "toolkit.telemetry.enabled";
-static const char kOldTelemetryPref[] = "toolkit.telemetry.enabledPreRelease";
 static const char kChannelPref[] = "app.update.channel";
 
 // clang-format off
@@ -3485,12 +3484,6 @@ Preferences::InitializeUserPrefs()
   // Don't set mCurrentFile until we're done so that dirty flags work properly.
   sPreferences->mCurrentFile = prefsFile.forget();
 
-  // Migrate the old prerelease telemetry pref.
-  if (!Preferences::GetBool(kOldTelemetryPref, true)) {
-    Preferences::SetBoolInAnyProcess(kTelemetryPref, false);
-    Preferences::ClearUser(kOldTelemetryPref);
-  }
-
   sPreferences->NotifyServiceObservers(NS_PREFSERVICE_READ_TOPIC_ID);
 }