remove unused includes

This patch removes includes that are not used. The removals were found with
include-what-you-use which checks if any of the symbols from a header is
in use.

src/test/test-json.c

@@ -21,7 +21,6 @@
 
 #include <math.h>
 
-#include "log.h"
 #include "util.h"
 #include "json.h"