Remove callcode in heading

Author: minaminao

docs/introduction-to-smart-contracts.rst

@@ -504,9 +504,9 @@ operations, loops should be preferred over recursive calls. Furthermore,
 only 63/64th of the gas can be forwarded in a message call, which causes a
 depth limit of a little less than 1000 in practice.
 
-.. index:: delegatecall, callcode, library
+.. index:: delegatecall, library
 
-Delegatecall / Callcode and Libraries
+Delegatecall and Libraries
 =====================================
 
 There exists a special variant of a message call, named **delegatecall**