crypto/tls: remove bookkeeping code from pHash function

crypto/tls: remove bookkeeping code from pHash function

Since copy function can figure out how many bytes of data to copy when
two slices have different length, it is not necessary to check how many
bytes need to copy each time before copying the data.

Change-Id: I5151ddfe46af5575566fe9c9a2648e111575ec3d
Reviewed-on: https://go-review.googlesource.com/71090
Reviewed-by: Filippo Valsorda <hi@filippo.io>
Run-TryBot: Filippo Valsorda <hi@filippo.io>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>