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

src/backend/libpq/be-secure-openssl.c

@@ -1075,7 +1075,7 @@ load_dh_buffer(const char *buffer, size_t len)
 	BIO		   *bio;
 	DH		   *dh = NULL;
 
-	bio = BIO_new_mem_buf(unconstify(char *, buffer), len);
+	bio = BIO_new_mem_buf(buffer, len);
 	if (bio == NULL)
 		return NULL;
 	dh = PEM_read_bio_DHparams(bio, NULL, NULL, NULL);