INT-2900 Remove Extraneous Debug Log

When referencing a `<gateway/>` from a `<service-activator/>`, a debug
log is written with a full stack trace.

This is because all methods on the proxy are considered candidates,
but one method (addAdvice) has multiple parameters and can't be
used to process a message.

Add code to detect a proxy that has no target object; if the Proxy
only has one interface, then use that as the targetClass.