Don't check for duplicate ids in cpp domain.

The proposed PR removes the check for duplicate ids in the cpp domain. This is done for the following reasons:

1. The check prevents target generation when the same node is rendered more than once. This happens when rendering matches during function overload resolution.

2. Duplicates are aslo diagnosed by TargetHandler (https://github.com/michaeljones/breathe/blob/e0aac07816081118ccbb372890c9d37f3715d22a/breathe/renderer/rst/doxygen/target.py#L19) as "Duplicate target detected: {id}" and Sphinx itself as "Duplicate ID: "{id}"".

Fixes issue https://github.com/michaeljones/breathe/issues/125.