cmd/dist: remove dead code for META_BUILDLET_HOST_TYPE

cmd/dist: remove dead code for META_BUILDLET_HOST_TYPE

This environment variable appears to never be used; according to
https://cs.opensource.google/search?q=META_BUILDLET_HOST_TYPE&ss=go&ssfr=1
this is the only reference to it in the whole Go project.

Change-Id: I6426a1d9e16441358c0e88daf4358112e659b2a9
Reviewed-on: https://go-review.googlesource.com/c/go/+/463741
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>