openpgp: remove TeeReader from packet.Read

This is a followup to issue 6927044. I *really* don't want to
break streaming for large encrypted data packets.

Removing the automatic re-parsing. OpaqueReader can be
used for recovering useful information from mangled/unsupported packets.

R=agl
CC=golang-dev
https://golang.org/cl/6944056
3 files changed