Remove attachmentservicemenu in favor of Purpose plugin

Summary: Since 5.48 Purpose has a FileItemActionPlugin that adds a "Send via Mail" action to Dolphin et al. It offers all the functionality of the KMail implementation, but respects the users prefered Email program. KDE Connect and BlueDevil dropped their custom actions in favor of Purpose plugins, too.

Test Plan: "Send as Email Attachment" action no longer appears in Dolphin. Can still send emails via "Share->Send via Email"

Reviewers: #kde_pim_kmail, mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D14811