Remove unused variable

Author: andrjohns

stan/math/prim/fun/hypergeometric_1F0.hpp

@@ -29,7 +29,6 @@ namespace math {
  */
 template <typename Ta, typename Tz, require_all_arithmetic_t<Ta, Tz>* = nullptr>
 return_type_t<Ta, Tz> hypergeometric_1F0(const Ta& a, const Tz& z) {
-  constexpr const char* function = "hypergeometric_1F0";
   check_less("hypergeometric_1F0", "abs(z)", std::fabs(z), 1.0);
 
   return boost::math::hypergeometric_1F0(a, z, boost_policy_t<>());