[dev.unified] cmd/compile: drop package height from Unified IR importer

CL 410342 removed package height, but still needs to keep writing out 0
for iexport for compatibility with existing importers. With Unified IR,
we don't have to, so get rid of the package height completely.

Change-Id: I84a285cbaddd7bb0833d45a24a6818231b4d2b71
Reviewed-on: https://go-review.googlesource.com/c/go/+/411014
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>

src/cmd/compile/internal/importer/ureader.go

@@ -162,8 +162,6 @@ func (r *reader) doPkg() *types2.Package {
 	}
 
 	name := r.String()
-	_ = r.Len() // was package height, but not necessary anymore.
-
 	pkg := types2.NewPackage(path, name)
 	r.p.imports[path] = pkg
 

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

@@ -292,7 +292,6 @@ func (r *reader) doPkg() *types.Pkg {
 	}
 
 	name := r.String()
-	_ = r.Len() // was package height, but not necessary anymore.
 
 	pkg := types.NewPkg(path, "")
 

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

@@ -328,7 +328,6 @@ func (pw *pkgWriter) pkgIdx(pkg *types2.Package) pkgbits.Index {
 		base.Assertf(path != "builtin" && path != "unsafe", "unexpected path for user-defined package: %q", path)
 		w.String(path)
 		w.String(pkg.Name())
-		w.Len(0) // was package height, but not necessary anymore.
 
 		w.Len(len(pkg.Imports()))
 		for _, imp := range pkg.Imports() {

src/go/internal/gcimporter/ureader.go

@@ -198,10 +198,8 @@ func (r *reader) doPkg() *types.Package {
 	}
 
 	name := r.String()
-	height := r.Len()
 
-	// Was: "pkg := types.NewPackageHeight(path, name, height)"
-	pkg, _ := types.NewPackage(path, name), height
+	pkg := types.NewPackage(path, name)
 	r.p.imports[path] = pkg
 
 	imports := make([]*types.Package, r.Len())