Remove Constant class

Constants are not a construct present in Clingo’s AST and were
unintentionally made part of anthem’s AST. This removes the unused
classes for clarity.

src/anthem/ASTCopy.cpp

@@ -103,13 +103,6 @@ Comparison prepareCopy(const Comparison &other)
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
-Constant prepareCopy(const Constant &other)
-{
-	return Constant(std::string(other.name));
-}
-
-////////////////////////////////////////////////////////////////////////////////////////////////////
-
 Function prepareCopy(const Function &other)
 {
 	return Function(std::string(other.name), prepareCopy(other.arguments));
@@ -293,11 +286,6 @@ struct FixDanglingVariablesInTermVisitor
 	{
 	}
 
-	template <class... Arguments>
-	void visit(Constant &, Arguments &&...)
-	{
-	}
-
 	template <class... Arguments>
 	void visit(Function &function, Arguments &&... arguments)
 	{

src/anthem/ASTUtils.cpp

@@ -150,10 +150,6 @@ struct CollectFreeVariablesVisitor
 		binaryOperation.right.accept(*this, variableStack, freeVariables);
 	}
 
-	void visit(Constant &, VariableStack &, std::vector<VariableDeclaration *> &)
-	{
-	}
-
 	void visit(Function &function, VariableStack &variableStack, std::vector<VariableDeclaration *> &freeVariables)
 	{
 		for (auto &argument : function.arguments)