[MJAVADOC-398] Do not add classes from build output directory to classpath

See http://jira.codehaus.org/browse/MJAVADOC-398

Project's classes on classpath can confuse javadoc tool in some
cases. The result can be either incorrect serialized-form.html
page or failure (in case of jdk8). See [this bug] for more details.

[this bug]: https://bugzilla.redhat.com/show_bug.cgi?id=1113877