cmd/link: remove unused flag -D (FlagDataAddr)

FlagDataAddr is a vestige from git commit 0cafb9ea (2008;
no Gerrit CL number).

It was never used but unfortunately setting it would
cause a spurious warning:

  warning: -D<value> is ignored because of -R0x1000

yet if -R was unset e.g. -R=0, the linker would crash
with a divide by zero runtime panic.

Fixes #28921

Change-Id: Ia910399bc269337a9a860f3a26cd48fae6e62724
Reviewed-on: https://go-review.googlesource.com/c/151021


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>