Comment out unsupported klee_is_symbolic

This takes a memory address as an argument, not a possibly-symbolic
value (which we do support). We probably need to extend the DeepState
API to support this robustly.

src/include/deepstate/Klee.h

@@ -55,8 +55,8 @@ static void klee_print_expr(const char *msg, ...) {
 /* TODO(joe): Implement */
 static uintptr_t klee_choose(uintptr_t n);
 
-/* TODO(joe): Implement */
-static unsigned klee_is_symbolic(uintptr_t n);
+/* Unsupported. */
+/* static unsigned klee_is_symbolic(uintptr_t n); */
 
 /* Unsupported. */
 /* static void klee_assume(uintptr_t condition); */