Revert "cmd/go: note when some Go files were ignored on no-Go-files errors"

This reverts commit eee727d0
(https://golang.org/cl/29113)

The " (.go files ignored due to build tags)" error message is not
always accurate.

Fixes #18396
Updates #17008

Change-Id: I609653120603a7f6094bc1dc3a83856f4b259241
Reviewed-on: https://go-review.googlesource.com/34662
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

api/go1.8.txt

@@ -176,7 +176,6 @@ pkg expvar, method (*Float) Value() float64
 pkg expvar, method (Func) Value() interface{}
 pkg expvar, method (*Int) Value() int64
 pkg expvar, method (*String) Value() string
-pkg go/build, type NoGoError struct, Ignored bool
 pkg go/doc, func IsPredeclared(string) bool
 pkg go/types, func Default(Type) Type
 pkg go/types, func IdenticalIgnoreTags(Type, Type) bool

src/go/build/build.go

@@ -439,16 +439,11 @@ func (ctxt *Context) ImportDir(dir string, mode ImportMode) (*Package, error) {
 // containing no buildable Go source files. (It may still contain
 // test files, files hidden by build tags, and so on.)
 type NoGoError struct {
-	Dir     string
-	Ignored bool // whether any Go files were ignored due to build tags
+	Dir string
 }
 
 func (e *NoGoError) Error() string {
-	msg := "no buildable Go source files in " + e.Dir
-	if e.Ignored {
-		msg += " (.go files ignored due to build tags)"
-	}
-	return msg
+	return "no buildable Go source files in " + e.Dir
 }
 
 // MultiplePackageError describes a directory containing
@@ -880,7 +875,7 @@ Found:
 		return p, badGoError
 	}
 	if len(p.GoFiles)+len(p.CgoFiles)+len(p.TestGoFiles)+len(p.XTestGoFiles) == 0 {
-		return p, &NoGoError{Dir: p.Dir, Ignored: len(p.IgnoredGoFiles) > 0}
+		return p, &NoGoError{p.Dir}
 	}
 
 	for tag := range allTags {

src/go/build/build_test.go

@@ -93,17 +93,6 @@ func TestEmptyFolderImport(t *testing.T) {
 	}
 }
 
-func TestIgnoredGoFilesImport(t *testing.T) {
-	_, err := Import(".", "testdata/ignored", 0)
-	e, ok := err.(*NoGoError)
-	if !ok {
-		t.Fatal(`Import("testdata/ignored") did not return NoGoError.`)
-	}
-	if !e.Ignored {
-		t.Fatal(`Import("testdata/ignored") should have ignored Go files.`)
-	}
-}
-
 func TestMultiplePackageImport(t *testing.T) {
 	_, err := Import(".", "testdata/multi", 0)
 	mpe, ok := err.(*MultiplePackageError)

src/go/build/testdata/ignored/ignored.go

-// +build alwaysignore
-
-package ignored