[CS17]Remove useless code(AsyncChannel) from ConnectivityService

The caller of AsyncChannel has been deleted in the last commit
(Remove legacy network factories), so the useless code of
AsyncChannel should also be deleted.

Bug: 170917042
Test: atest FrameworksNetTests
Change-Id: I6ce164fe5e3d592f1ed63753e8d3c0086bf61fa2