tests: stop validating neutronclient in neutron-debug tests

tests: stop validating neutronclient in neutron-debug tests

In neutronclient 4.0.0, any command executed triggers DeprecationWarning
on stderr (to be fixed by I77f168af92ae51ce16bed4988bbcaf7c18557727 and
a new client release including it).

The test cases assumed that if command is successful, it never writes to
stderr though. Making the test failing when using the latest client.

Instead of fixing the test class not to assume there is no output on
stderr, remove it because we are not meant to validate neutronclient in
neutron gate at all and should rely on the library as shipped. Client
should already have reasonable coverage for its CLI.

Change-Id: I6440445b80637a5a9f4de052cf5ea1fbd8dcf7d1
Closes-Bug: #1533638