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