phabricator: unconditionally pop `test_vcr` to fix debugcallconduit

Authored by Kwan.


phabricator: unconditionally pop test_vcr to fix debugcallconduit

11592ce6a711 / D8525 accidentally broke debugcallconduit in non-test scenarios
because it stopped popping test_vcr from kwargs unconditionally, so when
--test-vcr isn't set the empty string still gets passed down as the value of
test_vcr in kwargs. However unlike all the other commands debugcallconduit
doesn't have an **opts argument to receive it, so it aborts because of
invalid arguments.

Differential Revision: https://phab.mercurial-scm.org/D8852