refactor: remove unused, duplicated code in deprecate module (#14579)

* remove obsolete tests

 * remove unused deprecate API

 * make a warnOnce helper for the deprecate methods

 * misc. copyediting, e.g. variable names, whitespace

 * test that any deprecation warns once at most

 * use strict

lib/browser/api/app.js

@@ -44,7 +44,9 @@ const nativeFn = app.getAppMetrics
 app.getAppMetrics = () => {
   let metrics = nativeFn.call(app)
   for (const metric of metrics) {
-    deprecate.removeProperty(metric, 'memory', true)
+    if ('memory' in metric) {
+      deprecate.removeProperty(metric, 'memory')
+    }
   }
 
   return metrics