Revert "Notify bugsnag on 404 errors"

This reverts commit 2eec3d625a.

We started tracking 404 errors out of interest without an actual problem
to solve. Now we face rate-limiting in our Bugsnag account. And we
didn't use these 404 reports in the two years this code was active. We
don't even act on all 500 errors. So while our resources are so
constrained, let's keep our focus on the severe errors and user reports
and ignore the rest. 404 errors are mostly generated by vulnerability
scanners.

app/controllers/errors_controller.rb

@@ -4,11 +4,6 @@ class ErrorsController < ApplicationController
   layout "errors"
 
   def not_found
-    Bugsnag.notify("404") do |event|
-      event.severity = "info"
-
-      event.add_metadata(:request, :env, request.env)
-    end
     render status: :not_found, formats: :html
   end