do not override DCE settings if `-D doc-gen` is in place (closes #3939)

extra/CHANGES.txt

@@ -46,6 +46,7 @@
 	all : allowed variance on interface variables
 	all : allowed pattern matching on arrays if they are typed as Dynamic
 	all : allowed pattern matching on fields of parent classes
+	all : -D doc-gen no longer implies -dce no
 	flash/js: optimized haxe.ds.StringMap
 	neko : create output directory if it does not exist
 	js : inline Math methods and fields

filters.ml

@@ -1110,7 +1110,7 @@ let run com tctx main =
 	(* check @:remove metadata before DCE so it is ignored there (issue #2923) *)
 	List.iter (check_remove_metadata tctx) com.types;
 	(* DCE *)
-	let dce_mode = if Common.defined com Define.As3 || Common.defined com Define.DocGen then
+	let dce_mode = if Common.defined com Define.As3 then
 		"no"
 	else
 		(try Common.defined_value com Define.Dce with _ -> "no")