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.h

@@ -62,7 +62,6 @@ __attribute__ ((format (PRINTF_ATTRIBUTE_FORMAT, 5, 6)));
  */
 void log_init(void);
 void log_exit(void);
-int set_log_output(struct command_context *cmd_ctx, FILE *output);
 
 int log_register_commands(struct command_context *cmd_ctx);