Drop .arcconfig

It's been a while since we stopped used Phabricator for code review and
moved to GitLab, let's drop the vestigial `.arcconfig` file.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

.arcconfig

-{
-    "phabricator.uri" : "https://phabricator.apertis.org/",
-    "history.immutable": false,
-    "projects": "apertis-docker-images",
-    "default-reviewers": "#Reviewers"
-}