Fix message headers when permanently decrypting

* src/mail.cpp (Mail::decryptPermanently_o): Remove content type.
* src/mapihelp.cpp (mapi_set_transport_headers),
(mapi_set_content_type): New.

While the code allows to set the content type we just remove
it to avoid setting a wrong content type. I hope other
addins handle this properly.

The transport headers are not really used by Outlook after
a Mail is converted into MAPI.

This avoids that GpgOL itself tries to decrypt permanently
decrypted mails again.

• GnuPG-Bug-Id: T4718