clean_unused_images() function is redundant and wrong on preprocess.py file ?

[fangxu]

for example '08': ['001100', '005170'] , the 08 sequences has 4071 (5170-1100+1) pictures. the start number is still 0000000000.png and the end file's name is 0000004070.png. So the program shouldn't run clean_unused_images() function.