vk_rasterizer: Workaround bug in VK_EXT_vertex_input_dynamic_state

Workaround potential bug on Nvidia's driver where only updating high
attributes leaves low attributes out dated.

src/video_core/engines/maxwell_3d.h

@@ -305,10 +305,6 @@ public:
                 return (type == Type::SignedNorm) || (type == Type::UnsignedNorm);
             }
 
-            bool IsConstant() const {
-                return constant;
-            }
-
             bool IsValid() const {
                 return size != Size::Invalid;
             }