Docs: Clean up ENGINE mentions

Docs: Clean up ENGINE mentions

Engines are removed and the documentation should reflect that, even with
deprecated and legacy API.

Resolves: https://github.com/openssl/project/issues/1366

Signed-off-by: Norbert Pocs <norbertp@openssl.org>
Reviewed-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/29305)