[intro.execution] Remove unnecessary function from example (cplusplus#1084)

Author: hubert-reinterpretcast

source/intro.tex

@@ -1007,7 +1007,6 @@
 \begin{example}
 
 \begin{codeblock}
-void f(int, int);
 void g(int i) {
   i = 7, i++, i++;    // \tcode{i} becomes \tcode{9}