Remove own attachments when encrypting again

* src/mail.cpp (Mail::updateOOMData_o): Remove our own attachments.
(Mail::removeOurAttachments_o): Hide attachment remove warning.

The new attachment encryption code caused the gpgol_mime_structure
(MOSSTMPL) to be included in the encrypted attachment when encrypting
again for example when encrypting a draft or forwarding an encrypted
message.