Netflow input - not respecting "skip" in field definitions

[rich-graylog]

From a customer call summary:

Netflow 9 field definitions: When configured as skip it creates the field anyways with a generic name. Such as nf_field_235. You can drop these in a pipeline but it's another step that shouldn't be necessary if you're using custom field definitions.

We should properly handle and ignore these "Skip" fields if a user has defined them in their Netflow 9 definitions file.