[REF] Simplify code calculating the number of membership terms

[REF] Simplify code calculating the number of membership terms

We have 2 possibilities
1) there is a price set in use - the number of line items comes from the calculated line items
2) default priceset in use - it is a submitted value

The option to submit is only present in 2 so we know that if it is
submitted it should take precedence, otherwise it comes from the line item

This simplifies the code to do that clearly