feat: Don't download group messages unconditionally

[iequidoo]

There was a comment that group messages should always be downloaded to avoid inconsistent group state, but this is solved by the group consistency algo nowadays in the sense that inconsistent group state won't spread to other members if we send to the group.

Moving this simplification to a separate PR as suggested in https://github.com/chatmail/core/pull/7426#issuecomment-3597813059