CA-265469: Revert "CA-265469: start xapi-nbd from xapi"
CA-265469: Revert "CA-265469: start xapi-nbd from xapi"

Don't start xapi-nbd from xapi, because it will be path-activated by
systemd when the .pem certificate file appears that xapi-nbd needs for
TLS, which is enabled by default. When it was started by xapi, during
first boot the .pem file was generated by scripts/generate_ssl_cert
*after* the startup sequence finished when we got a management interface
& IP, and therefore xapi-nbd failed to start, because the .pem file it
required wasn't there.

This reverts commit 5a4fc0d132132b7be674c56b000a7332dee9f885.

srowe/xen-api,srowe/xen-api,srowe/xen-api,srowe/xen-api,srowe/xen-api