Stop having a separate flag for MUC stanza persistence.

[guusdk]

For personal archives, stanzas are always persisted if message archiving is enabled. For MUC archives, stanza persistence is controlled by an additional flag.

This is confusing. If message archiving is enabled, stanzas should, by default, be saved too.

When we do want to retain an option to skip storing stanzas, this needs to be applied to both personal as well as MUC archives in a consistent manner.