cmd/compile/internal/types2: remove Config.AllowTypeLists (cleanup)

This flag is not used anymore.

Change-Id: Id083ffa4f0a3051d3f1905a211db8acaaf525cf4
Reviewed-on: https://go-review.googlesource.com/c/go/+/355931
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Dan Scales <danscales@google.com>

src/cmd/compile/internal/noder/irgen.go

@@ -44,7 +44,6 @@ func checkFiles(noders []*noder) (posMap, *types2.Package, *types2.Info) {
 		GoVersion:             base.Flag.Lang,
 		IgnoreLabels:          true, // parser already checked via syntax.CheckBranches mode
 		CompilerErrorMessages: true, // use error strings matching existing compiler errors
-		AllowTypeLists:        true, // remove this line once all tests use type set syntax
 		Error: func(err error) {
 			terr := err.(types2.Error)
 			base.ErrorfAt(m.makeXPos(terr.Pos), "%s", terr.Msg)

src/cmd/compile/internal/types2/api.go

@@ -141,12 +141,6 @@ type Config struct {
 	// TODO(gri) Consolidate error messages and remove this flag.
 	CompilerErrorMessages bool
 
-	// If AllowTypeLists is set, the type list syntax is permitted
-	// in an interface in addition to the type set syntax.
-	// TODO(gri) Remove once type lists are no longer supported by
-	//           the parser.
-	AllowTypeLists bool
-
 	// If go115UsesCgo is set, the type checker expects the
 	// _cgo_gotypes.go file generated by running cmd/cgo to be
 	// provided as a package source file. Qualified identifiers