gen.go: copy all tablesXX.X.X.go versions to golang.org/x/net

Remove logic that copies only the most current tablesXX.X.X.go,
renaming it to tables.go. The golang.org/x/net/idna package
currently contains all the versioned files and no tables.go,
so this change makes gen.go consistent with the last export.

Change-Id: Ic2797b45ec998873651eda04ed56ada29788f0f0
Reviewed-on: https://go-review.googlesource.com/c/text/+/360380
Trust: Damien Neil <dneil@google.com>
Trust: Marcel van Lohuizen <mpvl@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/gen.go b/gen.go
index 04ff603..fb2fb94 100644
--- a/gen.go
+++ b/gen.go
@@ -256,12 +256,6 @@
 			filepath.Dir(file) != dirSrc {
 			return nil
 		}
-		if strings.HasPrefix(base, "tables") {
-			if !strings.HasSuffix(base, gen.UnicodeVersion()+".go") {
-				return nil
-			}
-			base = "tables.go"
-		}
 		b, err := ioutil.ReadFile(file)
 		if err != nil || bytes.Contains(b, []byte("\n// +build ignore")) {
 			return err