remove more unused references to ISurface

Change-Id: I2201f1ca2bb8f203a081d94a0134f798778dfbef
diff --git a/libs/gui/Surface.cpp b/libs/gui/Surface.cpp
index 0dfbf01..44d9b4b 100644
--- a/libs/gui/Surface.cpp
+++ b/libs/gui/Surface.cpp
@@ -502,8 +502,8 @@
     return NO_ERROR;
 }
 
-sp<ISurface> Surface::getISurface() const {
-    return mSurface;
+sp<IBinder> Surface::asBinder() const {
+    return mSurface!=0 ? mSurface->asBinder() : 0;
 }
 
 // ----------------------------------------------------------------------------