[debugger] remove step count parameter from prepare step.

test/mjsunit/debug-stepin-builtin-callback-opt.js

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Flags: --expose-debug-as debug
 
 Debug = debug.Debug
 
 var exception = null;
 var break_count = 0;
 
 function listener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
     print(event_data.sourceLineText());
     assertTrue(
         event_data.sourceLineText().indexOf(`Break ${break_count++}.`) > 0);
-    exec_state.prepareStep(Debug.StepAction.StepIn, 1);
+    exec_state.prepareStep(Debug.StepAction.StepIn);
   } catch (e) {
     exception = e;
   }
 };
 
 function replaceCallback(a) {
   return "x";                                   // Break 2.
 }                                               // Break 3.
 
 var re = /x/g;
 // Optimize the inner helper function for string replace.
 for (var i = 0; i < 10000; i++) "x".replace(re, replaceCallback);
 
 Debug.setListener(listener);
 debugger;                                       // Break 0.
 var result = "x".replace(re, replaceCallback);  // Break 1.
 Debug.setListener(null);                        // Break 4.
 
 assertNull(exception);