go/internal/gcimporter: cleanup test code pre Unified IR

go/internal/gcimporter: cleanup test code pre Unified IR

Same as CL 543315 did for cmd/compile.

Change-Id: I56a200664c92fe34d4b78c2dbf6ed56288e89e0e
Reviewed-on: https://go-review.googlesource.com/c/go/+/600675
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>