Add a note that you cannot use specific fields when you delete with predicate

[russorat]

https://docs.influxdata.com/influxdb/cloud/reference/syntax/delete-predicate/#limitations

As reported in this issue influxdata/influxdb#20704 and the related issues, it is not possible to delete a specific _field value in InfluxDB. We should update the docs to reflect this.

[aldas]

@russorat will deleting/dropping field be implemented in InfluxDB ever? I assume as data is packed into rows in a way that removing it not easily possible and therefore this feature would need excessive mount of work and change - and therefore it is not worth the effort of implementing it?

influxdata/influxdb#6150 is from 2016
influxdata/influxdb#20399 is 6 months old