helper/list: remove unused hlist_*

helper/list: remove unused hlist_*

The file list.h is taken from Linux and includes two similar
implementation of double linked lists:
- with single linked list's head (hlist_*), and
- with double linked list's head (list_*).
While the former offers a minor memory footprint improvement,
keeping two implementations makes harder for newbie developers
to approach them.

So far only the latter implementation has been used and no new
patches in gerrit is going to change that.

Drop the support for lists with single linked head.
It can be easily taken back from git history, if needed.

Change-Id: I420e5de38ab755fdfbeb2115538c61818308ec2b
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7567
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>