[COMETD-557] asyncContext not needed here.  fixes:

See previous pull here, which had other non-related commits: https://github.com/cometd/cometd/pull/23

JSONP transport throwing exception "Async not started"
ex:
Undertow.io:  2014-10-07 16:51:11,211 ERROR[io.undertow.request] UT005023: Exception handling request to /comet: java.lang.IllegalStateException: UT010018: Async not started
     at io.undertow.servlet.spec.HttpServletRequestImpl.getAsyncContext(HttpServletRequestImpl.java:972)
(cherry picked from commit ac38fd9)