cmd/link: delete unreachable hash collision check

cmd/link: delete unreachable hash collision check

This expression in readsym:

    dup != nil && len(dup.P) > 0 && strings.HasPrefix(s.Name, "gclocals·")

can never be true: if dup != nil, then s.Name is ".dup" (and this is not new:
the same broken logic is present in 1.4, at least). Delete the whole block.

Change-Id: I33b14d9a82b292116d6fd79d22b38e3842501317
Reviewed-on: https://go-review.googlesource.com/20970
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>