Remove always-true membership_assign

Remove always-true membership_assign

The combination of membership_assign and useForMember toggles whether the membership
information shows up. It shows up when we are not showing line items - in
which case it shows in the line item block instead - see
https://github.com/civicrm/civicrm-core/pull/28181

membership_assign is Always assigned when this receipt
is sent - hence it can go

useForMember winds up being a pseudonym of isShowLineItems
so this instance is replaced with that
- #28181 replaces some further instances