[dev.link] cmd/link: stop marking DWARF constant symbols

DWARF constant symbols were always marked and converted to
sym.Symbols when DWARF generation uses sym.Symbols. Now that the
DWARF generation uses the loader, no need to force-mark them.

Change-Id: Ia4032430697cfa901fb4b6d106a483973277ea0a
Reviewed-on: https://go-review.googlesource.com/c/go/+/226803


Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>