Removed outdated assembly descriptors

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpasyncclient/trunk@1531543 13f79535-47bb-0310-9956-ffa450edef68
HTTPASYNC-56: Removed method level synchronization in MinimalClientExchangeHandlerImpl and DefaultClientExchangeHandlerImpl. Instances of both classes are expected to be accessed by one thread at a time. Only #cancel method can be used safely by multiple threads concurrently.

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpasyncclient/trunk@1531544 13f79535-47bb-0310-9956-ffa450edef68