Remove implementation-defined base classes from template listings

[alandefreitas]

The templates are currently listing implementation-defined base classes:

These should be removed.

[vinniefalco]

There's only one base class:

template<class Req, class Res>
class basic_router : public detail::any_router
{

and I do not want the derivation to be visible, because detail should be a squelched namespace. That is, that it is extracted but not emitted.