Revert "Allow stepping when paused at isolate start."

runtime/vm/debugger.h

Index: runtime/vm/debugger.h
diff --git a/runtime/vm/debugger.h b/runtime/vm/debugger.h
index 79a620c7bcee6ee9f74c367b1ae5e966357e38da..faaf558ff6d492014a788a0365e2515b1d98d355 100644
--- a/runtime/vm/debugger.h
+++ b/runtime/vm/debugger.h
@@ -459,12 +459,6 @@ class Debugger {
 
   bool IsPaused() const { return pause_event_ != NULL; }
 
-  // Put the isolate into single stepping mode when Dart code next runs.
-  //
-  // This is used by the vm service to allow the user to step while
-  // paused at isolate start.
-  void EnterSingleStepMode();
-
   // Indicates why the debugger is currently paused.  If the debugger
   // is not paused, this returns NULL.  Note that the debugger can be
   // paused for breakpoints, isolate interruption, and (sometimes)