Fix clang-cl regression introduced by #282

[pdimov]

The sources conditioned on wstreambuf existence were mistakenly added to WSOURCES, which resulted in them being present in both libraries. This causes link errors with clang-cl.

This pull request removes the inadvertent addition, fixing the link errors.

Note that currently the clang-cl CI job still fails, but for reasons unrelated to this change and outside of our control (actions/runner-images#8125.) It's supposed to be fixed by the GHA team this week when they deploy the fixed windows-2022 image.

[Flamefire]

FTR (previously mentioned on Slack): I don't think this is a good (final) solution. The 2 files in question (xml_oarchive.cpp & utf8_codecvt_facet.cpp) are (as far as I can tell) not actually dependent on std::wstream but boost/archive/detail/utf8_codecvt_facet.hpp errors out when it is not available and xml_oarchive.cpp includes it transitively:

In file included from ./boost/archive/iterators/mb_from_wchar.hpp:31,
                 from ./boost/archive/impl/xml_oarchive_impl.ipp:28,
                 from libs/serialization/src/xml_oarchive.cpp:23:
./boost/archive/detail/utf8_codecvt_facet.hpp:13:2: error: #error "wide char i/o not supported on this platform"

The mb_from_wchar dependency is (only) required to load/save std::wstring/wchar_t-arrays.

Summary: Is it really worth excluding a whole class because a single method requires something? And: Is that requirement even correct (it looks like std::wstring is used, but std::wstream is required)?

And going further: As std::wstream is C++03 is it still worth having conditionals on that?
However that also means that this fix is sufficient as the 2 files will in practice now (almost) always be included in the build

@robertramey