MPL: Remove use of obsolete BOOST_NO_EXPLICIT_FUNCTION_TEMPLATE_ARGUMENTS

This macro is no longer defined for any compiler.
MPL: Remove use of obsolete BOOST_MPL_HAS_XXX_NO_EXPLICIT_TEST_FUNCTION
MPL: Remove use of obsolete BOOST_MPL_CFG_NO_APPLY_TEMPLATE

It was only defined for MSVC versions which are no longer supported.
MPL: Remove MSVC workaround for void templates.
MPL: Remove assert workaround for no-longer-supported MSVC.
MPL: Bump the MSVC compiler requirements
MPL: Remove obsolete files for old MSVC compilers.
MPL: Remove obsolete MSVC version checks.
MPL: Remove MSVC 6 and 7 early template instantiation workaround.
MPL: Remove use of obsolete macro.
MPL: Remove apply-wrap workaround for no-longer-supported MSVC
MPL: Remove workaround for no-longer-supported MSVC
MPL: Remove obsolete MSVC version checks.
MPL: Remove use of obsolete macro.
MPL: Remove use of obsolete macro.
MPL: Remove nntp_decl.hpp and obsolete includes.
MPL: Remove obsolete GCC version checks.