cmd/internal/{dwarf,obj}: stop substituting "" with pkgprefix

cmd/internal/{dwarf,obj}: stop substituting "" with pkgprefix

cmd/asm and cmd/compile now always create symbols with the appropriate
package prefixes, so cmd/internal/dwarf and cmd/internal/obj can stop
worrying about qualifying names itself.

Change-Id: I9aee5d759bf0d41a61722c777e7f66fce957e79e
Reviewed-on: https://go-review.googlesource.com/c/go/+/523338
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>