Drop support for depositing individual submissions

[jonasraoni]

Besides depositing full issues, the plugin also supports depositing individual submissions, which depends on the value of the object_type setting.

Problem:

• The setting isn't configurable, and its default value is to deposit issues. Therefore, it's basically alive just to support a legacy workflow, which will end up not being well tested and lead to zombie code.
• The current implementation attempts to group submissions in groups of 20 for each deposit. If the threshold isn't met, the deposit will never be created.

Notes:

• At the Preservation Network there are currently 571 deposits out of 137790 (0.4%), that perhaps are using this setting (they have neither an issue nor a volume number).
• Dropping this code will also allow merging the database entities of the plugin into a single one.
• This task should also cover the Drop "PublishedArticle" from the getObjectType() #63