configure arcanist for the git world

Problem

The documentation ("doc") repository has been converted to git but
arcanist does not yet understand how to interact with phabricator in
this world.

Solution

- remove the callsign "D" which should be inferable from having one of
  the cgit URLs as a remote
- remove "history.immutable" which was a workflow only required for
  subversion

.arcconfig

@@ -1,5 +1,3 @@
 {
-	"repository.callsign" : "D",
 	"phabricator.uri" : "https://reviews.freebsd.org/",
-	"history.immutable" : true
 }