refactor(api): use a keyset for highlight dicts

src/nvim/highlight_defs.h

@@ -46,18 +46,6 @@ typedef struct attr_entry {
   .hl_blend = -1, \
 }
 
-typedef struct {
-  char *bg_name;
-  char *fg_name;
-  char *sp_name;
-} HlAttrNames;
-
-#define HLATTRNAMES_INIT (HlAttrNames) { \
-  .bg_name = NULL, \
-  .fg_name = NULL, \
-  .sp_name = NULL, \
-}
-
 /// Values for index in highlight_attr[].
 /// When making changes, also update hlf_names below!
 typedef enum {