ERP55Form: drop a broken proxy field cache

When rendering a proxy field, 3 different fields can come in play:
1. the field to be rendered
3. the template field (i.e. not a proxy field) that knows how to render
2. possibly an intermediate proxy field that contains the value to render

What's difficult when rendering a proxy field is to take the above 3 fields
into account and this commit does it by creating a temporary field:
1. 'field' variable in TALES
2. the value
3. the code

Before this commit, 1 could be wrong.