Can I remove this truss cross-bracing?
Only the truss designer can say for sure whether they are required or not. A structural engineer might not be able to fully calculate the load transfers in the trusses, as they will only be able to estimate the joint strengths.

I'd hazard a guess that they are required or they'd not be there. From what I can tell from your photo (though it isn't quite clear), it looks like it could be one of the bracings that are required (in the UK at least) by the accepted standards for trusses (for an example, see here: NHBC pitched roof guide).

I'd be reluctant to remove it without first ensuring that you have sufficient equivalent bracing in place.