Drive by gardening of bv.go.

- Unexport the Bvec type, it is not used outside internal/gc.
  (machine translated with gofmt -r)
- Removed unused constants and functions.
  (driven by cmd/unused)

Change-Id: I3433758ad4e62439f802f4b0ed306e67336d9aba
Reviewed-on: https://go-review.googlesource.com/22602
Run-TryBot: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>