twister: Cleanup twister loggers

Removed logger.setLevel(...) from all twister modules, becasue
there should be only one place where twister loggers are configured,
and the log level should be set in that place.
This should be done in twister_main.py in setup_logging function.

Signed-off-by: Lukasz Fundakowski <lukasz.fundakowski@nordicsemi.no>