i#5857: Remove CONSTEXPR define

Created by: lihasgupta

Now that we do not support VS2013 and have C++11 everywhere we can remove this in clients/drcachesim/common/utils.h:

/* TODO(i#2924): Remove this and others like it once we stop supporting VS2013. */
#if defined(WINDOWS) && _MSC_VER < 1900
#    define CONSTEXPR const /* 'constexpr' not supported */
#else
#    define CONSTEXPR constexpr
#endif

This PR removes this.

Issue #5857