[REF] Convert previously shared function to non-static, remove unrelated code

[REF] Convert previously shared function to non-static, remove unrelated code

There is no facility for creating on_behalf on back office membership forms
so this code, copied from the shared function, does not relate.

The activity IS created by Contribution.create - the extra code was
just to hack it around to doing on_behalf

Likewise, the back office membership form does not collect
data on contribution custom fields or notes so removing these
also makes sense here.