internal/export/unicode: simplified generation

by using x/text’s infrastructure

Updates golang/go#27945

Change-Id: I070afd9cad4e367ac9085a66064f2bb47e5e225a
Reviewed-on: https://go-review.googlesource.com/c/text/+/154443
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

Author: mpvl

internal/export/unicode/doc.go

@@ -9,5 +9,5 @@ package unicode
 // standard packages to have non-standard imports, even if imported in files
 // with a build ignore tag.
 
-//go:generate go run gen.go -tables=all -output tables.go
+//go:generate go run gen.go -tables=all
 //go:generate mv tables.go $GOROOT/src/unicode