nixos/test-driver: rm global logger

We remove the global rootlog in favor of instantiating the logger as
required in the __init__.py and pass it down as a parameter (of our
AbstractLogger type).

nixos/lib/test-driver/test_driver/logger.py

@@ -307,6 +307,3 @@ class XMLLogger(AbstractLogger):
         self.log(f"(finished: {message}, in {toc - tic:.2f} seconds)")
 
         self.xml.endElement("nest")
-
-
-rootlog: CompositeLogger = CompositeLogger([TerminalLogger()])