exp/types, exp/gotype: remove exp/types

exp/types, exp/gotype: remove exp/types

The only code change is in exp/gotype/gotype.go.
The latest reviewed version of exp/types is now
exp/types/staging.

First step toward replacing exp/types with
exp/types/staging.

R=iant
CC=golang-dev
https://golang.org/cl/6819071