[SMS App] Disable sending attachments until we have Android support

Summary

Soft-remove support for any ways to instruct KDE Connect to send a message with an attachment.

It might still be possible for someone to use raw dbus to attempt to send an attachment. To them I say: "Please help with the Android-side patch to make this work, it's nearly there."

Android-side patch (needs some revisions): kdeconnect-android!205 (closed)

Test Plan

Launched SMS app, verified attachment button was missing

Verified that CLI returns an error when attempting to use the --attachment flag