basic/missing: drop _ETHTOOL_LINK_MODE_MAX

It was only used in one place, where we don't actually need it, and
it is too easy to forget to update it when adding new items to the table.
Let's just drop it.

src/basic/missing.h

@@ -1577,11 +1577,4 @@ enum ethtool_link_mode_bit_indices {
 #endif
 #endif
 
-/* Strictly speaking, this is not a missing field. Just for our convention. */
-#if !HAVE_ETHTOOL_LINK_MODE_FEC_NONE_BIT
-#define _ETHTOOL_LINK_MODE_MAX                       52
-#else
-#define _ETHTOOL_LINK_MODE_MAX                       (__ETHTOOL_LINK_MODE_LAST + 1)
-#endif
-
 #include "missing_syscall.h"