tests: remove unused macros

Since v0.6.2-7-g1d138acc ("Unify DATA and other frames in
nghttp2_outbound_item and save malloc()"), the macros are unused and the
builds fails on -Werror=unused-macros.

tests/nghttp2_session_test.c

@@ -36,10 +36,6 @@
 #include "nghttp2_test_helper.h"
 #include "nghttp2_priority_spec.h"
 
-#define OB_CTRL(ITEM) nghttp2_outbound_item_get_ctrl_frame(ITEM)
-#define OB_CTRL_TYPE(ITEM) nghttp2_outbound_item_get_ctrl_frame_type(ITEM)
-#define OB_DATA(ITEM) nghttp2_outbound_item_get_data_frame(ITEM)
-
 typedef struct {
   uint8_t buf[65535];
   size_t length;