xgl-generate.py: simplify _is_dispatchable()
xgl-generate.py: simplify _is_dispatchable()

Make use of object_in_params().  Remove unused _is_name_dispatchable().

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools