log: remove unused set_log_output()

The function set_log_output() has never been used after the drop
of eCos build with commit 39650e2273bc ("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