Test cleanup, deprecate ContributionSoft::retrieve

[eileenmcnaughton]

This removes the only call to ContributionSoft::retrieve that I could
find in core, hence I added a deprecation notice to the function.

There could be some extension uses - although it would never have
been supported to call this rather than the api

[civibot]

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[eileenmcnaughton]

thanks @seamuslee001