Remove OPENSSL_ALLOW_PROXY_CERTS.

One less random environment variable for us to be sensitive to. (We
should probably unwind all this proxy cert stuff. I don't believe they
are ever enabled.)

Change-Id: I74993178679ea49e60c81d8416e502cbebf02ec9
Reviewed-on: https://boringssl-review.googlesource.com/8948
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

crypto/x509/x509_vfy.c

@@ -588,12 +588,6 @@ static int check_chain_extensions(X509_STORE_CTX *ctx)
     } else {
         allow_proxy_certs =
             ! !(ctx->param->flags & X509_V_FLAG_ALLOW_PROXY_CERTS);
-        /*
-         * A hack to keep people who don't want to modify their software
-         * happy
-         */
-        if (getenv("OPENSSL_ALLOW_PROXY_CERTS"))
-            allow_proxy_certs = 1;
         purpose = ctx->param->purpose;
     }