Remove implicit IMAGE_TYPE suffix from local backend

[philipgian]

Commit fa08c92 introduced modular {src,dst} backends. The interface for src backend suports only the URL as argument. Local source backend also needs the IMAGE_TYPE to fallback to a deprecated behaviour, where the IMAGE_TYPE was implicitly added to the URL as suffix. As this is deprecated behaviour, I suggest we properly remove it (add changelog entry, remove relevant code, update the docs) instead of modifying the src backend interface.