gnrc_netif: remove superfluous GNRC_NETIF_FLAGS_MAC_*

They are also defined in `net/gnrc/netif/mac.h` and those are the ones
used by the current `gnrc_mac` implementations.

sys/include/net/gnrc/netif/flags.h

@@ -118,17 +118,6 @@ enum {
  */
 #define GNRC_NETIF_FLAGS_6LO_BACKBONE              (0x00000800U)
 
-/**
- * @brief   Mask for @ref gnrc_mac_tx_feedback_t
- */
-#define GNRC_NETIF_FLAGS_MAC_TX_FEEDBACK_MASK      (0x00006000U)
-
-/**
- * @brief   Flag to track if a transmission might have corrupted a received
- *          packet
- */
-#define GNRC_NETIF_FLAGS_MAC_RX_STARTED            (0x00008000U)
-
 /**
  * @brief   Network interface is configured in raw mode
  */