archiver: Remove cleanup goroutine from BufferPool

This isn't doing anything. Channels should get cleaned up by the GC when
the last reference to them disappears, just like all other data
structures. Also inlined BufferPool.Put in Buffer.Release, its only
caller.

internal/archiver/file_saver.go

@@ -76,7 +76,7 @@ func NewFileSaver(ctx context.Context, t *tomb.Tomb, save SaveBlobFn, pol chunke
 
 	s := &FileSaver{
 		saveBlob:     save,
-		saveFilePool: NewBufferPool(ctx, int(poolSize), chunker.MaxSize),
+		saveFilePool: NewBufferPool(int(poolSize), chunker.MaxSize),
 		pol:          pol,
 		ch:           ch,