Revert autocommit configuration (as is in DSpace 3.x)

This PR revert the DSpace 3.x autocommit settings for each solr core. Now with openSearcher set to true the indexer should be to work as in the past.

From solr documentation: openSearcher => Whether to open a new searcher when performing a commit. If this is false, the default, the commit will flush recent index changes to stable storage, but does not cause a new searcher to be opened to make those changes visible

https://jira.duraspace.org/browse/DS-1715