Don't require native CDB support to load {interwiki,trustedxff}.cdb

Both MediaWiki itself and the TrustedXFF extension use MediaWiki's CdbReader
wrapper, which provides a fully-functional PHP implementation for environments
that don't have native CDB support, like HHVM. This makes the checks for
function_exists( 'dba_open' ) unnecessary.

Bug: 69332
Change-Id: I313b09ffcc2f81dc061097b79b17910af1633119