selectTag has been hard coded to only understand the tag `go1` since
CL 6112060 which landed in 2012. The commit message asserted;

  Right now (before go1.0.1) there is only one possible tag,
  "go1", and I'd like to keep it that way.

Remove goTag and the unused matching code in selectTag.

Change-Id: I85f7c10f95704e22f8e8681266afd72bbcbe8fbd
Reviewed-on: https://go-review.googlesource.com/38112Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>