cmd/go: remove special case for prebuilt cgo library cache keys

cmd/go: remove special case for prebuilt cgo library cache keys

This was an oversight from CL 452457 that I noticed while
investigating #56889.

This change essentially undoes CL 335409, which is no longer needed
after CL 450739 because we no longer attempt to use cgo by default
when no C compiler is present.

Updates #47257.
Updates #40042.
Updates #47215.

Change-Id: I29c7ce777a9ec7ba5820dc1d836b12a61b86bc37
Reviewed-on: https://go-review.googlesource.com/c/go/+/452677
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>