ts-debian-di-install: Don't set runvars for netboot kernel+ramdisk as outputs

ts-debian-di-install: Don't set runvars for netboot kernel+ramdisk as outputs

Currently these runvars are either URLs provided by the definition
(e.g. make-flight) or output controller-relative paths created by the
execution (in the case where they aren't from the definition).

This wierd dual-semantics is confusing and wrong, and in particular is
broken if the test step is rerun (e.g. in standalone mode).

In the case where they are outputs only  these paths is information
only. The information is already available in the full logs so
dropping the runvars here merely removes the information from the
summary table. It's not so useful that this is an issue.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>