doc: remove unused code in call tracker example

This function appears to be unused, so remove it.

PR-URL: #44127
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

doc/api/assert.md

@@ -351,8 +351,6 @@ const tracker = new assert.CallTracker();
 
 function func() {}
 
-function foo() {}
-
 // Returns a function that wraps func() that must be called exact times
 // before tracker.verify().
 const callsfunc = tracker.calls(func, 2);
@@ -379,8 +377,6 @@ const tracker = new assert.CallTracker();
 
 function func() {}
 
-function foo() {}
-
 // Returns a function that wraps func() that must be called exact times
 // before tracker.verify().
 const callsfunc = tracker.calls(func, 2);