drm_hwcomposer: remove unused and confusing HWC_FB_BUFFERS definition.

BUG=none
TEST=builds

Change-Id: I5f143f93ebb82405320d4f0aaf03db4201380f8e
diff --git a/hwcomposer.cpp b/hwcomposer.cpp
index 7fec244..7b64bf2 100644
--- a/hwcomposer.cpp
+++ b/hwcomposer.cpp
@@ -46,7 +46,6 @@
 #include <utils/Trace.h>
 
 #define UM_PER_INCH 25400
-#define HWC_FB_BUFFERS 3
 
 namespace android {