[REM] fields: remove fields.Any, temporary artifact for ill-typed fields

This was added in master-apiculture at f1f16a8 to
permit special function fields that return
structured JSON-like data.
This is unnecessary and caused typing problems, for
example for the type field of ir.model.fields, or
when you decide to store them.

It is simpler to explicitly declare these fields
as fields.Char and have them serialize their results
to JSON strings, or to declate them as fields.Binary
and return any opaque data they want.