Fix #347: <shared_mutex>: Do we still need the _USING_V110_SDK71_ guard?

[StephanTLavavej]

Description

While we must continue to support msvcp140.dll running on Windows XP,
we don't need to support compiling our headers with the (removed)
v140_xp toolset and its corresponding old Windows SDK. Accordingly,
we can unconditionally define shared_mutex. (This is a C++17 feature,
but it was implemented before Standard modes, so it's not guarded by
_HAS_CXX17.)

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