Remove history.immutable from .arcconfig

The `history.immutable` setting prevents arcanist from updating
the commit messages with the Differential URL and therefore
makes updating patches awkward with a rebase workflow.

In case this new behaviour is not wanted the old one can be restored
by running `arc set-config --local history.immutable true`.

Test Plan: `arc diff --create HEAD^` adds the metadata now.

Reviewed By:	#phabric-admin, imp, lwhsu
Differential Revision: https://reviews.freebsd.org/D27971

.arcconfig

@@ -1,7 +1,6 @@
 {
 	"repository.callsign" : "S",
 	"phabricator.uri" : "https://reviews.freebsd.org/",
-	"history.immutable" : true,
 	"arc.land.onto.default": "main",
 	"arc.land.onto": ["main"]
 }