delete contributor_list field from Base_viewMetadata.

because we have no such generic property and this field can cause TypeError when saving.

product/ERP5/bootstrap/erp5_core/SkinTemplateItem/portal_skins/erp5_core/Base_viewMetadata.xml

@@ -95,7 +95,6 @@
                         <string>my_language</string>
                         <string>my_format</string>
                         <string>my_rights</string>
-                        <string>my_contributor_list</string>
                         <string>my_category_list</string>
                       </list>
                     </value>