I’m afraid you cannot, there is no such parameter. This is mandated by the JSON Encoding RFC, namespace-qualified member name must be used for all members of a top-level JSON object.
But you can post-process the result; if you are running the request from a command line, you can pass the result either to a tool such as the powerful JSON processor jq
your_request | python -c 'import json; import sys; \
json.dump({k.split(":")[1]: v for k, v in json.load(sys.stdin).items()}, \
fp=sys.stdout, indent=2)'
If you need to do that from within your code, I’m pretty sure your environment contains libraries that allow you to do the same.