Dune should no longer be tagged build-only

See ocaml/opam-repository#14266.

Author: aantron

odoc.opam

@@ -20,7 +20,7 @@ depends: [
   "astring" {build}
   "cmdliner" {build & >= "1.0.0"}
   "cppo" {build}
-  "dune" {build}
+  "dune"
   "fpath" {build}
   "ocaml" {>= "4.02.0"}
   "result" {build}