optimize encryption api

only one encryption key lookup in most cases instead of three
(has_key, get_key_size, get_key).

sql/sql_plugin_services.h

@@ -143,7 +143,6 @@ static struct encryption_keys_service_st encryption_keys_handler=
 {
   get_latest_encryption_key_version,
   has_encryption_key,
-  get_encryption_key_size,
   get_encryption_key
 };