Our install-dependencies script avoids installing asciidoctor if the
CI environment claims to have it already. But the only environment which
made use of this was Azure Pipelines, which went away in 6081d3898f (ci:
retire the Azure Pipelines definition, 2020-04-11). So this conditional
was effectively doing nothing. Let's clean it up.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>