tracing: Remove unused extern declaration tracing_map_set_field_descr()

Since commit 08d43a5 ("tracing: Add lock-free tracing_map"),
this is never used, so can be removed.

Link: https://lore.kernel.org/linux-trace-kernel/[email protected]

Cc: <[email protected]>
Signed-off-by: YueHaibing <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: YueHaibing

kernel/trace/tracing_map.h

@@ -272,10 +272,6 @@ extern u64 tracing_map_read_sum(struct tracing_map_elt *elt, unsigned int i);
 extern u64 tracing_map_read_var(struct tracing_map_elt *elt, unsigned int i);
 extern u64 tracing_map_read_var_once(struct tracing_map_elt *elt, unsigned int i);
 
-extern void tracing_map_set_field_descr(struct tracing_map *map,
-					unsigned int i,
-					unsigned int key_offset,
-					tracing_map_cmp_fn_t cmp_fn);
 extern int
 tracing_map_sort_entries(struct tracing_map *map,
 			 struct tracing_map_sort_key *sort_keys,