removes extra VaadinSession.setCurrent() from PushHandler. (#14222)

The setCurrent call on VaadinSession is not needed. the one extra call
can be saved because service.findVaadinSession will already set it.
Added a comment like it is done for UI (service.findUI will also set the
UI).

Change-Id: Ic24d922554d1316aae310813ef5d00a0bbfd418a