Add deprecation notice

[eileenmcnaughton]

Overview

I've been trying to figure out when this line would be hit but I just can't find a way to trigger
it - removing it would help a lot with this code but let's deprecate for a bit.

Before

LIne not hit as far as I can tell - $params['priceSetId'] has no way of being populated

After

Notice it it were to be hit

Technical Details

This code hasn't been touched for a while & lots has changed without stuff being cleaned up + functions that used to be shared are no longer so there seems to be a lot of unused stuff

Comments

[civibot]

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[yashodha]

@eileenmcnaughton can be merged.