Do not show attachment warning when printing

* src/mail.cpp (Mail::parsing_done): Silently remove
unencrypted attachments.

When printing we have already shown the warning. So we
should not show it again but silently remove any attachments
that are not hidden before our add_attachments. This
also fixes an issue that when printing sometimes the
child mails which are created for preview and print already
have the decrypted attachments.

• GnuPG-Bug-Id: T4560