layers: Make d/s CmdClearAttachments without d/s attachment warning
layers: Make d/s CmdClearAttachments without d/s attachment warning

This is pointless, but well-defined. Rework warning message to not
reference the VkClearAttachment::colorAttachment, as it's only
meaningful for color clears.

Signed-off-by: Chris Forbes <c3279c50043e18e9442c1ae071cafa0e7a4ca967@google.com>

KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers