src: remove cast for unsupported openssl

The cast is needed to build against OpenSSL 1.0.2, which master, 11.x, and 10.x no longer support.

To be clear, I mean don't support even when configured against an external openssl.

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