untag go1.2rc1

Looks like the dist tool really doesn't like tags.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/13354049

.hgtags

@@ -120,4 +120,3 @@ e570c2daeaca10663d36d6dee7f8d1d76e8f7b92 go1.1
 a7bd9a33067b3537351276e0178a045748ad046a go1.1.1
 414057ac1f1fc850957088e4c5e95cdbccd2d594 go1.1.2
 414057ac1f1fc850957088e4c5e95cdbccd2d594 release
-b8680c5c8a9f5df79f462aba6dfc11e537313e9f go1.2rc1