Replace `x[:]` where x is a slice with just `x`.

Found using https://go-critic.github.io/overview.html#unslice-ref

Change-Id: Ib0ee16e1d49b2a875b6b92a770049acc33208362
Reviewed-on: https://go-review.googlesource.com/123375
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>