When a node is looking for activities to process, there's no point testing the
'processing' column. This fixes cases leaving activities at 'processing=1'
forever (unless, of course, a cluster is killed and restarted with fewer nodes).
Also remove now useless cleanup at startup.

One known case leaving such activities is when the first commit of
dequeueMessage raises, which can happen, for example, during migration of
portal_types.

It seems the 'processing' column becomes useless for CMFActivity.

Acked-by: Vincent Pelletier

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@32089 20353a03-c40f-0410-a6d1-a30d3c3de9de