[XNIO-408] Drop check for StreamSourceChannel at Channels.drain(FileChannel, long, long)
[XNIO-408] Drop check for StreamSourceChannel at Channels.drain(FileChannel, long, long)

Signed-off-by: Flavia Rainone <1f6e33fa6229291b63ec96e1d0b155d164c84d68@redhat.com>

xnio/xnio,xnio/xnio