writer/hlsl: Do not emit (void) call_expr;

Just emit `call_expr;` instead.

Fixed: tint:1259
Change-Id: I84a2976d82f891e9263f3e11e5774fcb01ea09bd
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/67381
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

test/statements/assign/phony/call.wgsl.expected.hlsl

@@ -4,6 +4,6 @@ int f(int a, int b, int c) {
 
 [numthreads(1, 1, 1)]
 void main() {
-  (void) f(1, 2, 3);
+  f(1, 2, 3);
   return;
 }