Delete duplicate `prov:type` `nidmStatisticalMap`

As suggested by @nicholsn, we do not need to have a duplicate
`prov:type` attribute as `StatisticalMap` is a parent of
`TStatisticalMap` in the [spreadsheet of
terms](https://docs.google.com/spreadsheets/d/16pC2cDsdxlzv2CzlNMtStqt5-
xHwDEsU6MjZVxWhrE4/edit?usp=sharing).