[REF] Remove setting on unused variables

[REF] Remove setting on unused variables

The removed line sets membershipValues[x]['frequency_interval']
and membershipValues[x]['frequency_duration']

However I can see no evidence these are ever used anymore. The membershipValues
array is an array of values to pass to the membership create action which
does not reference them at all.

They ARE referenced from the legacyProcessRecurringContribution as
they defined the frequency of these - but they take the value set in
formValues which is not removed

Removing this one line opens up quite a lot more simplification and
I can't see any evidence it plays a function these days