elementjob: stop calculating cache size

Calculating the cache size on the child_process_data() is racy, as this
runs in the main process which does not obtain exclusive access to the
cache resource.

This calculation also seems unnecessary, as it will triggered when
BuildQueue.done() is called.

buildstream/_scheduler/jobs/elementjob.py

@@ -109,13 +109,11 @@ class ElementJob(Job):
         data = {}
 
         workspace = self._element._get_workspace()
-        artifact_size = self._element._get_artifact_size()
-        cache_size = self._element._get_artifact_cache().calculate_cache_size()
-
         if workspace is not None:
             data['workspace'] = workspace.to_dict()
+
+        artifact_size = self._element._get_artifact_size()
         if artifact_size is not None:
             data['artifact_size'] = artifact_size
-        data['cache_size'] = cache_size
 
         return data

buildstream/_scheduler/queues/queue.py

@@ -300,8 +300,6 @@ class Queue():
         # Update values that need to be synchronized in the main task
         # before calling any queue implementation
         self._update_workspaces(element, job)
-        if job.child_data:
-            element._get_artifact_cache().cache_size = job.child_data.get('cache_size')
 
         # Give the result of the job to the Queue implementor,
         # and determine if it should be considered as processed