BUG/MEDIUM: server: clear dynamic srv on delete from proxy id/name trees

BUG/MEDIUM: server: clear dynamic srv on delete from proxy id/name trees

Do not leave deleted server in used_server_id/used_server_addr backend
trees. This might lead to crashes if a deleted server is used through
these trees.

At this moment, dynamic servers are only added in used_server_id if they
have a fixed id. They are never inserted in used_server_addr as this
code is missing. So these new delete instructions are noop. However, a
fix will be provided soon to insert properly all dynamic servers in both
used_server_id and used_server_addr trees so the deletion counterpart
will be mandatory in the CLI server delete handler.

This must be backported to 2.4.