[py-tx] Deprecate SignalExchangeAPI.fetch_value_merge

[Dcallies]

This API was intended to support a fancy data storage model that we don't actually need if we split up downloading from the API and converting to signals, like we eventually did.

https://github.com/facebook/ThreatExchange/blob/main/python-threatexchange/threatexchange/exchanges/signal_exchange_api.py#L127

Next steps:

1. Should be marked as deprecated in the comments and marked for removal in 2.x.x
2. Existing callsites should be removed

Any existing implementations that convert records into None to signal a deletion should instead be moved into the fetch() method instead.