[d3d12] Avoid panic on instance drop

[hakolao]

Connections
Attempts to fix #4604

Description
register_exception_handler is only called when InstanceFlags::VALIDATION are present. Thus unregister should also be only called then.

    if instance_flags.contains(wgt::InstanceFlags::VALIDATION) {
        //...
        super::exception::register_exception_handler();
    }

Testing
Tested on one of my sample projects...

Checklist

• Run cargo fmt.
• Run cargo clippy. If applicable, add:
  • --target wasm32-unknown-unknown
  • --target wasm32-unknown-emscripten
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.

[nical]

Nice!

[nical]

Looks like you need to remove that bitflags::Flags import.