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/main.zig

@@ -1,10 +0,0 @@
-const std = @import("std");
-const testing = std.testing;
-
-export fn add(a: i32, b: i32) i32 {
-    return a + b;
-}
-
-test "basic add functionality" {
-    try testing.expect(add(3, 7) == 10);
-}