[GPU] Change texture load cbuffer to push constants

Simplify the code, eliminating the need for supporting requesting cbuffers
for anything other than guest draw command execution.

src/xenia/gpu/vulkan/vulkan_texture_cache.h

@@ -141,7 +141,6 @@ class VulkanTextureCache final : public TextureCache {
   enum LoadDescriptorSetIndex {
     kLoadDescriptorSetIndexDestination,
     kLoadDescriptorSetIndexSource,
-    kLoadDescriptorSetIndexConstants,
     kLoadDescriptorSetCount,
   };