DECODER: Some cleanups, and aligning with OSSL_ENCODER

DECODER: Some cleanups, and aligning with OSSL_ENCODER

Mostly source nits, but also removing a couple of OSSL_DECODER_PARAM
macros that are never used or even make sense.

Also, some function names weren't quite consistent.  They were made a
bit more consistent in the OSSL_ENCODER API, now we bring that back to
OSSL_DECODER.

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/12873)