Remove redundant configuration in d6_field migration

This is really low priority, but the d6_field migration has some outdated configuration. The process pipeline for determining the field type first calls the field_type process plugin, then skip_on_empty. The field_type plugin extends StaticMap (and has for some time), which means it will skip the row by default if mapping fails. Therefore, having skip_on_empty in the process pipeline is unnecessary. The d7_field migration already knows this, so I think the two migrations should be consistent.