[server] Get rid of unused MembershipRowToSave.subscription parameter

Summary: This isn't used anywhere so I'm getting rid of it to make the code simpler.

Test Plan: Flow

Reviewers: palys-swm

Reviewed By: palys-swm

Subscribers: KatPo, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D1022