log: remove unused set_log_output()

The function set_log_output() has never been used after the drop
of eCos build with commit 39650e2273 ("ecosboard: delete
bit-rotted eCos code") in 2012.

Drop it!

Change-Id: I070b688061776c7ced5db18f738d78a4a7623726
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7164
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins

src/helper/log.c

@@ -295,12 +295,6 @@ void log_exit(void)
 	log_output = NULL;
 }
 
-int set_log_output(struct command_context *cmd_ctx, FILE *output)
-{
-	log_output = output;
-	return ERROR_OK;
-}
-
 /* add/remove log callback handler */
 int log_add_callback(log_callback_fn fn, void *priv)
 {