Drop superfluous 'global' statements from command.py (#621)

I don't see any reason that these statements should be necessary and
find their presence confusing.

colcon_core/command.py

@@ -86,7 +86,6 @@ def register_command_exit_handler(handler):
 
     :param handler: The callable
     """
-    global _command_exit_handlers
     if handler not in _command_exit_handlers:
         _command_exit_handlers.append(handler)
 
@@ -113,7 +112,6 @@ def main(*, command_name='colcon', argv=None):
     :param list argv: The list of arguments
     :returns: The return code
     """
-    global _command_exit_handlers
     try:
         return _main(command_name=command_name, argv=argv)
     except KeyboardInterrupt:
@@ -126,7 +124,6 @@ def main(*, command_name='colcon', argv=None):
 
 
 def _main(*, command_name, argv):
-    global colcon_logger
     # default log level, for searchability: COLCON_LOG_LEVEL
     colcon_logger.setLevel(logging.WARNING)
     set_logger_level_from_env(
@@ -387,8 +384,6 @@ def create_subparser(parser, cmd_name, verb_extensions, *, attribute):
       selected verb
     :returns: The special action object
     """
-    global colcon_logger
-
     # list of available verbs with their descriptions
     verbs = []
     for name, extension in verb_extensions.items():