Remove output variables from compute regression tests

Output variables in GLCompute shaders is not supported in Vulkan.
Recent upgrade of spirv-tools revealed this problem.

Test/hlsl.attribute.expression.comp

@@ -5,11 +5,9 @@ uniform int bound;
 #define BAR 2
 
 [numthreads(2+2, 2*3, (1+FOO)*BAR)]
-float4 main() : SV_TARGET
+void main()
 {
     [unroll(5*2 + 1) ]
         for (int x=0; x<bound; ++x)
             ;
-    
-    return float4(0,0,0,0);
 }