Remove out of date comment from the end of set_devices_thread()

The comment became completely unnecessary with the transfer of
mount_info and fstab_info into separate Mount_Info module by commit:
    63ec73dfda
    Split mount_info and fstab_info maps into separate Mount_Info module

It was never necessary to clear one of the mappings at the end of the
device refresh because it was reloaded at the start of the next device
refresh anyway and it is only a small amount of memory.

src/GParted_Core.cc

@@ -379,8 +379,6 @@ void GParted_Core::set_devices_thread( std::vector<Device> * pdevices )
 		}
 	}
 
-	//clear leftover information...	
-	//NOTE that we cannot clear mountinfo since it might be needed in get_all_mountpoints()
 	set_thread_status_message("") ;
 	g_idle_add( (GSourceFunc)_mainquit, NULL );
 }