cmd/go: don't print deprecation notice for 'go get exe'

It's difficult for module authors to provide installation instructions
that work in both Go 1.15 and 1.16. We'll wait until 1.17 to print a
deprecation warning for installing executables with 'go get'.

Fixes #42885

Change-Id: I835b447e83e760f48fd664e8a117749e0cb59f83
Reviewed-on: https://go-review.googlesource.com/c/go/+/274552


Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>