crypto/tls: drop macFunction abstraction

Since we dropped SSLv3, there is only one MAC scheme, and it doesn't
need any state beyond a keyed HMAC, so we can replace the macFunction
with the hash.Hash it wraps.

Pointed out by mtp@.

Change-Id: I5545be0e6ccb34a3055fad7f6cb5f628ff748e9f
Reviewed-on: https://go-review.googlesource.com/c/go/+/251859


Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Trust: Roland Shoemaker <roland@golang.org>
Trust: Filippo Valsorda <filippo@golang.org>