BUG/MINOR: ssl: Clear the ckch instance when deleting a crt-list line

BUG/MINOR: ssl: Clear the ckch instance when deleting a crt-list line

When deleting a crt-list line through a "del ssl crt-list" call on the
CLI, we ended up free'ing the corresponding ckch instances without fully
clearing their contents. It left some dangling references on other
objects because the attache SSL_CTX was not deleted, as well as all the
ex_data referenced by it (OCSP responses for instance).

This patch can be backported up to branch 2.4.

(cherry picked from commit 23cab33b67dd9dc76de936f47dfa23b1a8ee40e5)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>