The jboss-osgi-http bundle is the provider of the HttpService and must among other packages import/export javax.servlet. In OSGi it is perfectly valid to have more than one exporter of the same package in the system. It is the job of the resolver to generate the wirings such that all the class spaces are consistent.
In your case, one exporter of javax.servlet is the javaee.api module - another is jboss-osgi-http. Your webapp is not an OSGi bundle and hence can only wire to javax.servlet available from the javaee.api module.