Do not hide attachments when they are nested in a message part which is itself an attachment

I introduced a bug in commit c430463; the flags which control part display were
not properly trimmed when recursively descending into the nested MIME parts.
This had an unfortunate side effect that an attached message/rfc822 which itself
contained an attached PDF file would hide the PDF file from display.

This is a pretty critical problem -- hiding message parts which are there is
never a good thing :(.

Thanks to Thomas Lübking for review.

REVIEW: 112291