Remove superfluous `service_variant` parameter
Remove superfluous `service_variant` parameter

This code looks and feels like it was pulled from `edx-platform`.
`service_variant` looks like a switch between `cms` and `lms`.

In our configuration, this defaults to `certs`; I'm going to assume
that's a reasonable value for everyone.

Stanford-Online/openedx-certificates,Stanford-Online/openedx-certificates,stvstnfrd/edx-certificates,stvstnfrd/edx-certificates