Documentation typo fix in EVP_EncryptInit.pod

Remove duplicate declaration of `EVP_CIPHER_key_length` in the synopsis.

CLA: trivial

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5792)