[REF] simplify retrieval of existing membership on membership renewal form

[REF] simplify retrieval of existing membership on membership renewal form

After stepping through the code I determined that if membershipID is passed into getContactMembership then it either
1) returns the membership id much like membership.get but with is_current_member calculated or
2) returns the owner membership in a similar format

However, the renewal form is not visible for a non-owner membership so getting the owner membership is cruft
from another form & only 1 applies. Calling the api is much more transparent