cmd/compile: remove vestigial importpath symbol logic
The object file format now has an explicit section for tracking which
packages were imported, so we don't need to write out importpath
symbols for all directly imported packages anymore.
However, keep the logic for writing out individual importpath symbols,
because it's still relevant to runtime type descriptor generation.
Change-Id: I184ff320e894ba43ca0f8a3d2678e4b2bbbe6da5
Reviewed-on: https://go-review.googlesource.com/c/go/+/523875
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com>