cmd/go/internal/load: remove a special case for "unsafe"

cmd/go/internal/load: remove a special case for "unsafe"

We had a special case to zero out the Target field for package
"unsafe", which is not imported from a normal object file.

As of CL 449376 that special case has been folded into go/build's
logic for setting the PkgObj field, so the special case in
cmd/go/internal/load has become redundant.

(Noticed while investigating CL 449376.)

Updates #47257.
Updates #56687.

Change-Id: I1668123aa6230097aa75e55380d3e2c7937c4b64
Reviewed-on: https://go-review.googlesource.com/c/go/+/449515
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>