src: remove unnecessary call to SSL_get_mode

SSL_set_mode() bit-ORs its argument into the current mode, its not necessary for to do it ourself (though it doesn't cause harm).

See:

• https://www.openssl.org/docs/man1.1.0/ssl/SSL_get_mode.html
• https://github.com/nodejs/node/blob/0ce615c4af/deps/openssl/openssl/ssl/ssl_lib.c#L2176-L2177

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines