ubifs-utils: Adapt commit.c in libubifs

ubifs-utils: Adapt commit.c in libubifs

Adapt commit.c in libubifs, compared with linux kernel implementations:
 1. Remove debug related implementations(eg. dbg_check_old_index), debug
    functions are not needed by fsck, because fsck will check fs in
    another way.
 2. Remove ubifs background committing related implementations, there
    will be no background ubifs threads in fsck.

Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>