Simplify / Fix attachment collection

* src/mimedataprovider.cpp (mime_context): Remove redundant
collect_attachment.
(t2body): Remove collect_attachment artifact. Infer from
crypto data / bodies / nesting level.

The old collect attachment handling rooted in the fact that
the old parser collected everything as attachments, even
the body. Now that we collect the body differently we don't
need to collect_attachment this way. We now collect
every nested parts that are not inline text as attachments
if they are not crypto_data.