BUG/MINOR: acme: don't unlink from acme_ctx_destroy()

BUG/MINOR: acme: don't unlink from acme_ctx_destroy()

Unlinking the acme_ctx element from acme_ctx_destroy() requires to have
the element unlocked, because MT_LIST_DELETE() locks the element.

acme_ctx_destroy() frees the data from acme_ctx with the ctx still
linked and unlocked, then lock to unlink. So there's a small risk of
accessing acme_ctx from somewhere else. The only way to do that would be
to use the `acme challenge_ready` CLI command at the same time.

Fix the issue by doing a mt_list_unlock_link() and a
mt_list_unlock_self() to unlink the element under the lock, then destroy
the element.

This must be backported in 3.2.