language: remove "may change" notice

Package should be pretty stable now.
No backwards incompatible changes expected after the currently
deprecated functionality has been removed.

Change-Id: I02e91082a9d90c0ed527d67ba00b36226dfe265e
Reviewed-on: https://go-review.googlesource.com/16869
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

language/language.go

@@ -100,9 +100,6 @@
 //
 // BCP 47 - Tags for Identifying Languages
 // http://tools.ietf.org/html/bcp47
-//
-// NOTE: This package is still under development. Parts of it are not yet
-// implemented, and the API is subject to change.
 package language // import "golang.org/x/text/language"
 
 // TODO: Remove above NOTE after: