libstdc++: [_GLIBCXX_INLINE_VERSION] Cleanup 
gnu-versioned-namespace.ver

     Remove expressions for symbols in std::__detail::__8 namespace, 
they are obsolete since
     version namespace applies only at std:: level, not at sub-levels.

     libstdc++-v3/ChangeLog:

             * config/abi/pre/gnu-versioned-namespace.ver: Remove 
obsolete std::__detail::__8
             symbols.

Tested under Linux x86_64.

Ok to commit ?

François