cmd/link: remove -W option

cmd/link: remove -W option

The -W option has not worked since Go 1.3.  It is not documented.  When
it did work, it generated useful output, but it was for human viewing;
there was no reason to write a script that passes the -W option, so it's
unlikely that anybody is using it today.

Change-Id: I4769f1ffd308a48324a866592eb7fd79a4cdee54
Reviewed-on: https://go-review.googlesource.com/15701
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>