BUILD: ssl: kill a build warning introduced by BoringSSL compatibility

BUILD: ssl: kill a build warning introduced by BoringSSL compatibility

A recent patch to support BoringSSL caused this warning to appear on
OpenSSL 1.1.0 :
   src/ssl_sock.c:3062:4: warning: statement with no effect [-Wunused-value]

It's caused by SSL_CTX_set_ecdh_auto() which is now only a macro testing
that the last argument is zero, and the result is not used here. Let's
just kill it for both versions.

Tested with 0.9.8, 1.0.0, 1.0.1, 1.0.2, 1.1.0. This fix may be backported
to 1.7 if the boringssl fix is as well.