ENCODER: Drop OSSL_ENCODER_PARAM_INPUT_TYPE

ENCODER: Drop OSSL_ENCODER_PARAM_INPUT_TYPE

This was a poor substitute for using the name of the decoder implementation,
and since there is functionality to get the latter now, this parameter
can be dropped.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15570)