[REF] Simplify activity import validation

[REF] Simplify activity import validation

There are 2 types of validation on importing
1) the correct fields have been mapped
2) the specific row has all the required fields

These generally live in different places
- field mapping is validated on the MapField::formRule validation function
- the specific row is validated in the (badly named) 'summary' function.

However, the second function is also doing some validation of the
mapping, checking that either activity_type_id or label is mapped
& ditto activity_date_time. This duplicates the MapField rule
and is hard to read, while making a larger cleanup difficult