Vulkan: Don't destroy the device's pipeline cache on MSAA mode change

The user could switch back again, and this would mean this data would be
lost. Disk space is cheap, and it's not going to be much.

Source/Core/VideoBackends/Vulkan/ObjectCache.h

@@ -123,10 +123,6 @@ public:
   // otherwise for a cache hit it will be true.
   std::pair<VkPipeline, bool> GetPipelineWithCacheResult(const PipelineInfo& info);
 
-  // Wipes out the pipeline cache, use when MSAA modes change, for example
-  // Also destroys the data that would be stored in the disk cache.
-  void ClearPipelineCache();
-
   // Saves the pipeline cache to disk. Call when shutting down.
   void SavePipelineCache();