refactor(string_view_base): remove hash_value

Since boostorg/core@92f6cfb in Boost.Core, we have been getting "error: use of undeclared identifier 'hash_value'" because hash_value is not part of the interface of core::string_view.

hash_value has been removed from core::string_view because it's redundant. Boost containers don't need it and it does not enable std containers.

include/boost/url/grammar/string_view_base.hpp

@@ -857,16 +857,6 @@ class string_view_base
 
     //--------------------------------------------
 
-    /** Return the hash of this value
-    */
-    friend
-    std::size_t
-    hash_value(
-        string_view_base const& s) noexcept
-    {
-        return hash_value(s.s_);
-    }
-
     /** Format a string to an output stream
      */
     BOOST_URL_DECL