ITS#7639 fix crash in config_delete

Additional fix to 41352ea3
The overlay must be deleted from the backend before the
callback can execute. In particular, it must be done before
the threadpool is unpaused.
3 jobs for master in 32 minutes and 15 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Build
passed build-gnutls-mit #1501

00:31:28

passed build-no-threads-no-slapd #1499

00:03:28

passed build-openssl-heimdal #1500

00:32:14