[debugger] remove step count parameter from prepare step.

test/mjsunit/es6/debug-step-into-regexp-subclass.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 --harmony-regexp-subclass
 
 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 customSplit() {
   return "x";              // Break 2.
 }                          // Break 3.
 var o = {};
 o[Symbol.split] = customSplit;
 
 Debug.setListener(listener);
 debugger;                  // Break 0.
 var result = "".split(o);  // Break 1.
 Debug.setListener(null);   // Break 4.
 
 assertEquals("x", result);
 assertNull(exception);