cmd/go: delete GOPATH-mode get

cmd/go: delete GOPATH-mode get

We've decided to keep basic GOPATH mode running
for trees that already exist, but GOPATH-mode get is
being removed. It is old and not useful and probably
full of security holes. See #60915 for more details.

Fixes #60915.

Change-Id: I9db4c445579bf0b79f6543624602652555b66c1d
Reviewed-on: https://go-review.googlesource.com/c/go/+/518775
Auto-Submit: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>