adb: remove incorrect use of RTTI.

We were dynamic_casting to UsbConnection to check for USB connections,
but the actual type was a BlockingConnectionAdapter wrapping a
UsbConnection, with the result that unplugging an inaccessible (due to
permissions) device on Linux wouldn't make the device go away.

Test: manual
Change-Id: Icb4acea5fd3c3baa9691698686213e122e898e4a