Simplify logging of mavsdk_server output

[EricPedley]

Motivation

In #585 I described how logging output from the child process for mavsdk_server at only debug level hides useful errors from the user.

Summary

This PR removes the logic for redirecting the subprocess stdout to the debugger and results in the mavsdk_server child process's stdout and stderr going straight to the main stdout/stderr.

[EricPedley]

What about making it an option?

Yeah this is a good idea. I added another parameter to System.connect that lets users turn off this logging.

[JonasVautherin]

I think we should try to fix what was done in #370 (if it is indeed broken) instead :thinking:

[julianoes]

@EricPedley sorry, I didn't merge this but I hope what I did in https://github.com/mavlink/MAVSDK-Python/pull/781 - released in 3.9.0 - works for you. If you could test it and report back whether it works for you, that would be fantastic.