remove .lst before rename in im2rec.py (#3958)

fout should be closed before doing anything, if it is not "WindowsError: [Error 32]" will occur.
fname should be removed before renaming fname+".tmp" to fname, if it is not "WindowsError: [Error 183]" will occur.
But why here must rewrite the lst file?

PS, My OS is Windows 10.

tools/im2rec.py

@@ -144,6 +144,8 @@ def write_worker(q_out, fname, working_dir):
             print('time:', cur_time - pre_time, ' count:', count)
             pre_time = cur_time
         count += 1
+    fout.close()
+    os.remove(fname)
     os.rename(fname+'.tmp', fname)
 
 def parse_args():