tests: Bluetooth: Remove setting a unused define

The underlying Kconfig setting NET_BUF_USER_DATA_SIZE is not used anywhere,
so this define is removed here.

Signed-off-by: Christian Taedcke <christian.taedcke@lemonbeat.com>
diff --git a/tests/bluetooth/controller/mock_ctrl/include/kconfig.h b/tests/bluetooth/controller/mock_ctrl/include/kconfig.h
index 6a77583..2eddf40 100644
--- a/tests/bluetooth/controller/mock_ctrl/include/kconfig.h
+++ b/tests/bluetooth/controller/mock_ctrl/include/kconfig.h
@@ -182,4 +182,3 @@
 #define CONFIG_BT_BUF_ACL_TX_COUNT 7
 #define CONFIG_BT_BUF_ACL_TX_SIZE 251
 #define CONFIG_BT_CTLR_RX_BUFFERS 7
-#define CONFIG_NET_BUF_USER_DATA_SIZE 8