Remove support for snappy compression.

Remove support for snappy compression.

LZ4 is using less CPU at similar performance, and it is easier to
build and support for binary installs (as it does not require C++
and a C++ runtime).  Since it was never supported in any formally
released OpenVPN version, just drop it again.

This leaves in the compression opcode for Snappy for documentation
purposes.

trac #617

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <1444494889-28925-1-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10251