[boost] "Remove obsolete MSVC checks" and _MSC_VER

One general note about these changes. This: #if defined( _MSC_VER ) && _MSC_VER >= 1310 doesn't mean "compiler is MS VC++ 7.1". It means "compiler claims to support MSVC 7.1's features." Many Windows compilers define _MSC_VER for compatibility. I'm not positive that the removal of the _MSC_VER >= part would break something nowadays, but if it does, this would be why. I'd have left these alone; it's not clear what purpose removing the check is intended to serve, apart from breaking code. Actual MSVC-specific code is guarded by BOOST_MSVC and not _MSC_VER.