Update element set code once certain tickets are resolved

[jimsafley]

Once the following tickets are resolved:

• https://omeka.org/trac/ticket/1101
• https://omeka.org/trac/ticket/972

remove the admin_append_to_files_form hook and it's callback; and use insert_element_set() instead of creating the Scripto element set from scratch in ScriptoPlugin::install().

[jimsafley]

The above tickets were migrated to GitHub:

• omeka/Omeka#74
• omeka/Omeka#34