chore(server): remove 'client store expiration'

Judging by the discussion in PR karma-runner#1040, where this option was added, and the
change in socket.io's Client.prototype.destroy function (which no longer
includes a timeout), I feel pretty confident we can remove this option

lib/server.js

@@ -263,15 +263,6 @@ var createSocketIoServer = function(webServer, executor, config) {
     // avoid destroying http upgrades from socket.io to get proxied websockets working
     'destroyUpgrade': false,
 
-    // TODO(juliemr): This is no longer an option since socket.io 0.9 - figure
-    // out if there's something that should replace it.
-    //
-    // socket.io has a timeout (15s by default) before destroying a store (a data structure where
-    // data associated with a socket are stored). Unfortunately this timeout is not cleared
-    // properly on socket.io shutdown and this timeout prevents karma from exiting cleanly.
-    // We change this timeout to 0 to make Karma exit just after all tests were executed.
-    'client store expiration': 0,
-
     path: (config.urlRoot[0] === '/' ? '' : '/') + config.urlRoot + 'socket.io'
   });