[Feature request] Drop field

Due to inattention, I have two fields, `rx_pkt` and `rx_packets`, each with a few million measurements, and I only need either of those.

Is there an equivalent to the following SQL to get rid of a single field in InfluxDB?

``` sql
alter table traffic drop `rx_pkt`, drop `tx_pkt`
```

Start by examining how InfluxDB stores fields and handles schema changes, then look for existing query-language or database alteration entry points related to the SQL example. Done means defining and implementing a supported way to remove one or more fields without removing the measurement, with tests covering the requested behavior.

Written by the indexing model from the issue text.