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