Remove obsolete unconstify()
This is no longer needed as of OpenSSL 1.1.0 (the current minimum
version). LibreSSL made the same change around the same time as well.
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://www.postgresql.org/message-id/
20463f79-a7b0-4bba-a178-
d805f99c02f9%40eisentraut.org