Remove remnants of an old, no longer applicable readback optimization

GPU/Vulkan/FramebufferManagerVulkan.cpp

@@ -305,10 +305,6 @@ VkImageView FramebufferManagerVulkan::BindFramebufferAsColorTexture(int stage, V
 	}
 }
 
-void FramebufferManagerVulkan::UpdateDownloadTempBuffer(VirtualFramebuffer *nvfb) {
-	// Nothing to do here.
-}
-
 void FramebufferManagerVulkan::BlitFramebuffer(VirtualFramebuffer *dst, int dstX, int dstY, VirtualFramebuffer *src, int srcX, int srcY, int w, int h, int bpp) {
 	if (!dst->fbo || !src->fbo || !useBufferedRendering_) {
 		// This can happen if they recently switched from non-buffered.