Remove external friend duplicates

While evaluating #924, we also noticed external friends are sometimes rendered more than once. This needs to be fixed.

For instance, note how std::hash<url_view> is duplicated in the documentation of boost::urls::url_view:

Friends

Name	Description
`boost::urls::params_encoded_base`	Common functionality for containers
`boost::urls::params_base`	Common functionality for query parameter containers
`boost::urls::url_view_base`	Common functionality for containers
`std::hash<url_view>`
`boost::urls::params_encoded_base`	Common functionality for containers
`boost::urls::params_base`	Common functionality for query parameter containers
`boost::urls::url_view_base`	Common functionality for containers
`std::hash<url_view>`