pgcrypto: Remove non-OpenSSL support

pgcrypto: Remove non-OpenSSL support

pgcrypto had internal implementations of some encryption algorithms,
as an alternative to calling out to OpenSSL.  These were rarely used,
since most production installations are built with OpenSSL.  Moreover,
maintaining parallel code paths makes the code more complex and
difficult to maintain.

This patch removes these internal implementations.  Now, pgcrypto is
only built if OpenSSL support is configured.

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://www.postgresql.org/message-id/flat/0b42f1df-8cba-6a30-77d7-acc241cc88c1%40enterprisedb.com