Removed 'mutable' member of variant_callable

In the old code, both this and the const version were part of a union, which meant both got intialized with a value.
Now, the mutable variable was always nullptr. Since we cannot pass a non-const callable ptr to the variant ctor, we
need to use const_cast.

src/formula/variant.hpp

@@ -98,26 +98,20 @@ class variant
 		return value_cast<game_logic::variant_callable>()->get_callable();
 	}
 
-	game_logic::formula_callable_ptr mutable_callable() const
-	{
-		must_be(VARIANT_TYPE::TYPE_CALLABLE);
-		return value_cast<game_logic::variant_callable>()->get_callable_mutable();
-	}
-
 	template<typename T>
 	T* try_convert() const
 	{
 		if(!is_callable()) {
 			return nullptr;
 		}
 
-		return dynamic_cast<T*>(mutable_callable().get());
+		return dynamic_cast<T*>(const_cast<game_logic::formula_callable*>(as_callable().get()));
 	}
 
 	template<typename T>
 	T* convert_to() const
 	{
-		T* res = dynamic_cast<T*>(mutable_callable().get());
+		T* res = dynamic_cast<T*>(const_cast<game_logic::formula_callable*>(as_callable().get()));
 		if(!res) {
 			throw type_error("could not convert type");
 		}

src/formula/variant_private.cpp

@@ -66,7 +66,6 @@ std::string variant_decimal::to_string_impl(const bool sign_value) const
 
 variant_callable::variant_callable(const formula_callable* callable)
 	: callable_(callable)
-	, mutable_callable_(nullptr) // FIXME
 {}
 
 std::string variant_callable::get_serialized_string() const

src/formula/variant_private.hpp

@@ -272,11 +272,6 @@ class variant_callable : public virtual variant_value_base
 		return callable_;
 	}
 
-	formula_callable_ptr get_callable_mutable()
-	{
-		return mutable_callable_;
-	}
-
 	virtual std::string string_cast() const override
 	{
 		return "(object)";
@@ -296,7 +291,6 @@ class variant_callable : public virtual variant_value_base
 
 private:
 	const_formula_callable_ptr callable_;
-	formula_callable_ptr mutable_callable_;
 };