Revert "Fix Half NaN definition and test."

This reverts commit c770746d70.

Eigen/src/Core/arch/Default/GenericPacketMathFunctionsFwd.h

@@ -70,15 +70,8 @@ EIGEN_DEFINE_FUNCTION_ALLOWING_MULTIPLE_DEFINITIONS
 EIGEN_UNUSED
 Packet pcos_float(const Packet& x);
 
-/** \internal \returns sqrt(x) for complex types */
-template<typename Packet>
-EIGEN_DEFINE_FUNCTION_ALLOWING_MULTIPLE_DEFINITIONS
-EIGEN_UNUSED
-Packet psqrt_complex(const Packet& x);
-
 template <typename Packet, int N> struct ppolevl;
 
-
 } // end namespace internal
 } // end namespace Eigen