function remove(source, destination) {
  const sourceCode = source.getCode();
  const destinationCode = destination.getCode();
  const transform = transforms[sourceCode][destinationCode];
  delete transforms[sourceCode][destinationCode];
  if (isEmpty(transforms[sourceCode])) {
    delete transforms[sourceCode];
  }
  return transform;
}
Unregisters the conversion function to convert coordinates from the source
projection to the destination projection.  This method is used to clean up
cached transforms during testing.

@param {import("./Projection.js").default} source Source projection.
@param {import("./Projection.js").default} destination Destination projection.
@return {import("../proj.js").TransformFunction} transformFn The unregistered transform.