multihost: remove MultihostUtility.used

This attribute was replaced by private _mh_utility_used and there
should be no reason to make this attribute public. Once any method
or @Property is accessed it is already set to True so it does not
have any information for the outside caller.

pytest_mh/_private/multihost.py

@@ -917,9 +917,6 @@ def __init__(self, host: HostType) -> None:
         self.logger: MultihostLogger = self.host.logger
         """Multihost logger."""
 
-        self.used: bool = False
-        """Indicate if this utility instance was already used or not within current test."""
-
         self.artifacts: set[str] = set()
         """
         List of artifacts that will be automatically collected at specific