Vulkan: Don't save pipeline cache if shader cache is disabled

We still create a pipeline cache object, since that speeds up driver's
creation of pipelines at runtime. However, we should not save it.

Source/Core/VideoBackends/Vulkan/ObjectCache.h

@@ -163,7 +163,8 @@ public:
   std::string GetDiskCacheFileName(const char* type);
 
 private:
-  bool CreatePipelineCache(bool load_from_disk);
+  bool CreatePipelineCache();
+  bool LoadPipelineCache();
   bool ValidatePipelineCache(const u8* data, size_t data_length);
   void DestroyPipelineCache();
   void LoadShaderCaches();