cue: remove FileSet from API

Change-Id: Idc52c4a07cc2ddfaaa217b30050a14e14584655d
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2123
Reviewed-by: Marcel van Lohuizen <mpvl@google.com>
diff --git a/internal/protobuf/parse.go b/internal/protobuf/parse.go
index b81bd5c..0298db3 100644
--- a/internal/protobuf/parse.go
+++ b/internal/protobuf/parse.go
@@ -596,8 +596,7 @@
 
 		case "(cue.val)":
 			// TODO: set filename and base offset.
-			fset := token.NewFileSet()
-			expr, err := parser.ParseExpr(fset, "", o.Constant.Source)
+			expr, err := parser.ParseExpr("", o.Constant.Source)
 			if err != nil {
 				failf("invalid cue.val value: %v", err)
 			}