libstdc++: Remove std::constract_violation from versioned namespace

GCC expects this type to be in std namespace directly.

libstdc++-v3/ChangeLog:

     * include/experimental/contract:
     Remove _GLIBCXX_BEGIN_NAMESPACE_VERSION/_GLIBCXX_END_NAMESPACE_VERSION.

It does fix 29 g++.dg/contracts in gcc testsuite.

Ok to commit ?

François