loader: Remove warning on missing implicit layer
loader: Remove warning on missing implicit layer

If the Implicit Layer registry location was missing, validation
would throw a warning.  This is not correct behavior as many systems will
not have Implicit layers.  Modify so the warning/error is only thrown if
either a driver manifest path, or an explicit layer path.

Change-Id: Ideccc730e0b1c613c4cc58c5f64ae31ba19bc05c

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