Remove the conversion to bool from json_view

explicit operator bool meant "refers to a value", which silently differs
from what a basic_json converts to. Use !v.is_discarded() instead.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/json_view.hpp

@@ -156,12 +156,6 @@ class basic_json_view
         return type() == value_t::discarded;
     }
 
-    /// false for discarded views
-    explicit operator bool() const noexcept
-    {
-        return m_node != nullptr;
-    }
-
     //////////////
     // capacity //
     //////////////