Docs: Update ENGINE_add.pod to reflect the engine removal

Docs: Update ENGINE_add.pod to reflect the engine removal

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)