Remove the envvar hack to enable proxy cert processing

When the proxy cert code was initially added, some application authors
wanted to get them verified without having to change their code, so a
check of the env var OPENSSL_ALLOW_PROXY_CERTS was added.

Since then, the use of this variable has become irrelevant, as it's
likely that code has been changed since, so it's time it gets removed.

Reviewed-by: Tim Hudson <tjh@openssl.org>