filer: remove unnecessary code

- Remove close(s.cacheStartedCh) in destroy() - the channel is only used
  for one-time synchronization, closing it provides no benefit
- Remove task := task loop variable capture - Go 1.22+ fixed loop variable
  semantics, this capture is no longer necessary (go.mod specifies Go 1.24.0)

weed/filer/reader_at.go

@@ -267,7 +267,6 @@ func (c *ChunkReadAt) doReadAt(ctx context.Context, p []byte, offset int64) (n i
 		g.SetLimit(concurrency)
 
 		for _, task := range tasks {
-			task := task // capture for closure
 			g.Go(func() error {
 				// Read directly into the correct position in the output buffer
 				copied, readErr := c.readChunkSliceAtForParallel(gCtx, p[task.bufferStart:task.bufferEnd], task.chunk, task.chunkOffset)

weed/filer/reader_cache.go

@@ -223,7 +223,6 @@ func (s *SingleChunkCacher) destroy() {
 	if s.data != nil {
 		mem.Free(s.data)
 		s.data = nil
-		close(s.cacheStartedCh)
 	}
 }