misc/dist: don't ship cmd/api

cmd/api is a tool to prevent the Go developers from breaking
the Go 1 API promise. It has no utility to end users and
doesn't run on arbitrary packages (it's always been full of
hacks for its bespoke type checker to work on the standard
library)

Robert's in-progress rewrite depends on the go.tools repo for
go/types, so we won't be able to ship this tool later
anyway. Just remove it from binary distributions.

A future change to run.bash can conditionally build & run
cmd/api, perhaps automatically fetching go/types if
necessary. I assume people don't want to vendor go/types into
a private gopath just for cmd/api.

I will need help with run.bat.

R=golang-dev, adg, dsymonds, rsc
CC=golang-dev
https://codereview.appspot.com/12316043