cmd/go: remove support for -buildmode=shared

It never worked in module mode (or with a read-only GOROOT).
A proposal to drop it was filed (and approved) in
https://golang.org/issue/47788.

Fixes #47788

Change-Id: I0c12f38eb0c5dfe9384fbdb49ed202301fa4273d
Reviewed-on: https://go-review.googlesource.com/c/go/+/359096


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>