cmd/internal/obj: remove unused code

cmd/internal/obj: remove unused code

Following on from CL 14350, remove the remaining dead code from data.go.

Also leave a TODO to be addressed later (with a unit test) to reduce
the overhead of SymGrow.

Change-Id: Iebad775b1280b54b89e87a3a073ca8af19a8bfba
Reviewed-on: https://go-review.googlesource.com/14359
Run-TryBot: Dave Cheney <dave@cheney.net>
Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>