crypto: remove unused C++ parameter in sign/verify

Removes code in node_crypto.cc in Sign::SignFinal and
Verify::VerifyFinal which allowed to convert between buffers and
strings based on given encodings. The code is unused as crypto.js
only passes in and expects buffers and does the conversion itself.
The encoding parameter was removed from both methods.

PR-URL: https://github.com/nodejs/node/pull/12397


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>