pdns, dnsdist: enable luajit
With the recent replacement of luajit by the openresty branch, pdns and dnsdist no longer need exceptions for luajit on aarch64.
For pdns, this MR simply unblocks the build.
The dnsdist port has logic to use luajit where possible, and lua otherwise. With this MR, that logic is no longer used, but I have left it in place in case we need it for other architectures.