runtime: remove unused stack.go

We're never going to use stack.go for gccgo.  Although a build tag
keeps it from being built, even having it around can be confusing.
Remove it.

Change-Id: If23657d400608226c3aaf70d37808b957f0c0528
Reviewed-on: https://go-review.googlesource.com/40774
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
1 file changed