Move UNUSED out of C++11-dependent #ifdef

[The code says](https://github.com/dds-bridge/dds/pull/4/files#diff-f077def8d1d46ce3832e5ddabf7f524dL55):

    // In C++11 UNUSED(x) is explicitly provided

But that isn't true.  **nullptr** is explicitly provided, however, in C++11.

What is true is that there are a bunch of codebases that define this exact macro somewhere.  But it should not be necessary to write it conditionally for MS compilers.

In a technical sense, the name USE may be better.  The reason is because UNUSED might end up being a lie--there's nothing stopping one from later adding a reference to the so-called UNUSED thing, no error will be caused.