VideoCommon/Statistics: Remove unused DECSTAT macro

This isn't used anywhere, so it can be removed. This also potentially
fixes an underlying compilation error waiting to happen, given DECSTAT
could have potentially been used, someone disables statistics (for
whatever reason), then gets a compilation error due to the #else case
not containing an empty definition of DECSTAT.

Source/Core/VideoCommon/Statistics.h

@@ -73,7 +73,6 @@ extern Statistics g_stats;
 
 #ifdef STATISTICS
 #define INCSTAT(a) (a)++;
-#define DECSTAT(a) (a)--;
 #define ADDSTAT(a, b) (a) += (b);
 #define SETSTAT(a, x) (a) = (int)(x);
 #else