hwc: resolve compiler warnings

- remove unused variables.
- fix incorrect castings.

Change-Id: I7661edb4539a3ffc7d4aedbb9f633bc5ba1dc149
diff --git a/libhwcomposer/hwc_virtual.h b/libhwcomposer/hwc_virtual.h
index 502aa3b..26f401f 100644
--- a/libhwcomposer/hwc_virtual.h
+++ b/libhwcomposer/hwc_virtual.h
@@ -75,11 +75,11 @@
     // instantiates mdpcomp, copybit and fbupdate objects and initialize those
     // objects for virtual display during virtual display connect. This function
     // is no-op for V4L2 design
-    virtual void init(hwc_context_t *ctx) {};
+    virtual void init(hwc_context_t *) {};
     // Destroys mdpcomp, copybit and fbupdate objects and for virtual display
     // during virtual display disconnect. This function is no-op for V4L2 design
-    virtual void destroy(hwc_context_t *ctx, size_t numDisplays,
-                       hwc_display_contents_1_t** displays){};
+    virtual void destroy(hwc_context_t *, size_t ,
+                       hwc_display_contents_1_t** ) {};
 };
 
 }; //namespace