[AirTunes] - don't handle the flush callback from shairplay - we handled

it wrong and it isn't needed for us (actually this even fixes a race
condition).

Reference:

http://forum.xbmc.org/showthread.php?tid=201355

and upstream comment (line notes):

https://github.com/juhovh/shairplay/commit/fda63ad40b874933e2342a1f2f3d6f4b642d12ff