Fix #4971: NoMethodError when trying to display some modreports.

Delete modreports for hard-deleted comments. There were a total of six
invalid modreports for deleted comments.

script/fixes/096_delete_invalid_moderation_reports.rb

@@ -0,0 +1,14 @@
+#!/usr/bin/env ruby
+
+require_relative "base"
+
+with_confirmation do
+  CurrentUser.scoped(User.system) do
+    ModerationReport.find_each do |report|
+      if report.invalid? && report.errors[:model] == ["must exist"]
+        puts "destroying modreport ##{report.id}"
+        report.destroy!
+      end
+    end
+  end
+end