Cleanup: delete .arcconfig

This no longer works without developer.blender.org

.arcconfig

-{
-	"project_id" : "Blender Dev Tools",
-	"conduit_uri" : "https://developer.blender.org/",
-	"phabricator.uri" : "https://developer.blender.org/",
-	"git.default-relative-commit" : "origin/master",
-	"arc.land.update.default" : "rebase"
-}