Realm: remove the mutex in MachineImpl

[eddy16112]

The mutex referred in this topic is https://gitlab.com/StanfordLegion/legion/-/blob/master/runtime/realm/machine_impl.h#L135.

After switching to use allgatherv for machine announcement, the set methods of MachineImpl except for add_subscription are called from RuntimeImpl::finish_configure sequentially, and the get methods are not modifying any internal data, so it is not necessary to protect them with the mutex.