cmd/go: drop PackageInternal.Pkgdir (unused)

Change-Id: Iab8f0d201780bd571541a6806f071e883a553d35
Reviewed-on: https://go-review.googlesource.com/56286Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>

src/cmd/go/internal/load/pkg.go

@@ -93,7 +93,6 @@ type PackagePublic struct {
 type PackageInternal struct {
 	// Unexported fields are not part of the public API.
 	Build        *build.Package
-	Pkgdir       string               // overrides build.PkgDir
 	Imports      []*Package           // this package's direct imports
 	Target       string               // installed file for this package (may be executable)
 	Pkgfile      string               // where package will be (or is already) built or installed