Make the server unlink itself

To clean up the device, the client executed "adb shell rm" once the
server was guaranteed to be started (after the connection succeeded).

This implied to track whether the installation state, and failed if an
additional tunnel was used in "forward" mode:
<https://github.com/Genymobile/scrcpy/issues/386#issuecomment-453936034>

Instead, make the server unlink itself on start.