cmd/api: update api checker to use go/types from std repo

cmd/api: update api checker to use go/types from std repo

The old code checked out a specific version of go/types from the
x/tools repo. With go/types being part of the std repo, this is
not necessary anymore.

Also, for the same reason, the api tool is now built like any
other regular command. There's no need to build it for each run.
Removed the respective +build tags.

Change-Id: I5088e4867223d676957084c24651ec05452ac495
Reviewed-on: https://go-review.googlesource.com/8564
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>