remove Zig example in favor of mach/gpu one

`mach/gpu` has an example of how to use the bindings: https://github.com/hexops/mach/tree/main/gpu/examples

The example here is largely duplicative of that, and doesn't currently build. It would need to be updated
to reflect the latest Dawn example code.

Instead, let's keep `mach/gpu-dawn` just scoped to building Dawn with Zig. `mach/gpu` will deal with using it.

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

gpu-dawn/src/dawn/dummy.zig

@@ -0,0 +1 @@
+pub fn main() void {}