Fixed the problem with multiple sounds not playing at the same time

The problem is visible on iOS 8 if someone wants to play multiple sounds at the same time. When one sound ends (is released) AVSession (that is a shared instance per app) is set not active, causing other playing sounds to stop playing.

My simple solution was to remove all calls where the shared app. AVSession is inactivated. Should not cause memory leaks since it's a shared instance.

Could be that this AVSession should be released when app is paused.