Remove obsolete MSVC check from pragma guard

git grep -h -B1 "^#\s*pragma once" | grep -v pragma | sort | uniq

is now clean.

[SVN r85952]

include/boost/range/numeric.hpp

@@ -19,7 +19,7 @@
 // accompanying file LICENSE_1_0.txt or copy at
 // http://www.boost.org/LICENSE_1_0.txt)
 
-#if defined(_MSC_VER) && _MSC_VER >= 1000
+#if defined(_MSC_VER)
     #pragma once
 #endif