hwc: resolve compiler warnings

- remove unused variables.
- fix incorrect castings.

Change-Id: I7661edb4539a3ffc7d4aedbb9f633bc5ba1dc149
diff --git a/libhwcomposer/hwc_virtual.cpp b/libhwcomposer/hwc_virtual.cpp
index 600ba8d..dbe6bb8 100644
--- a/libhwcomposer/hwc_virtual.cpp
+++ b/libhwcomposer/hwc_virtual.cpp
@@ -62,7 +62,7 @@
         ctx->mMDPComp[dpy]->reset();
 }
 
-void HWCVirtualVDS::destroy(hwc_context_t *ctx, size_t numDisplays,
+void HWCVirtualVDS::destroy(hwc_context_t *ctx, size_t /*numDisplays*/,
                        hwc_display_contents_1_t** displays) {
     int dpy = HWC_DISPLAY_VIRTUAL;