cmd/go: remove the special case for "unsafe" in importFromModules

The comment for this special case claims:
> There's no directory for import "C" or import "unsafe".

However, there clearly is a directory for "unsafe" in
GOROOT/src/unsafe, and all of our integration tests seem to pass
without this special case. As far as I can tell, it's just confusing.

Also note that the internal/goroot package explicitly considers
package "unsafe" to be in the standard library; see CL 137435.

For #36460

Change-Id: Ib857d18f731a7f3c911c1bd116a34e3a9b3d74a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/315412


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>