Revert "Workaround for Atmosphere race condition (#13528)" (#14534)

This reverts commit 7f9ea1c9159c8362e6a309b630cbb59a4dae72dd.
The problem is fixed in Atmosphere 2.2

Conflicts:
	server/src/com/vaadin/server/communication/PushRequestHandler.java

Change-Id: I04a1b0c87927beb9b2048fee1ec95cd17af53a28