Remove dataset field from logs package

[jen-huang]

With the changes done to change dataset.* fields to data_stream.* in elastic/kibana#74414 and elastic/beats#20420, we should remove the dataset field definition in logs package:

https://github.com/elastic/integrations/blob/master/packages/log/dataset/log/manifest.yml#L19
https://github.com/elastic/integrations/blob/master/packages/log/dataset/log/agent/stream/stream.yml.hbs#L7

While grepping, I found one other instance of a similar field in netflow package, but I'm not sure if this needs to be removed as it's part of ECS fields:
https://github.com/elastic/integrations/blob/master/packages/netflow/dataset/log/fields/ecs.yml#L940

[ruflin]

I'm trying to fix this in #234

For the ECS fields in netflow: This is expected.

[jen-huang]

Closing as #234 is merged.