nilfs2: delete unnecessary condition in load_segment_summary

This is a trivial patch to remove unnecessary condition.

load_segment_summary() checks crc of segment_summary OR crc of whole
log data blocks based on boolean argument full_check.  However,
callers of the function pass only 1 as full_check, which means only
whole log data blocks checking code is running all the time.

This patch deletes the condition and full_check argument and also
deletes enum 'NILFS_SEG_FAIL_CHECKSUM_SEGSUM' and corresponding case
clause, for it is nolonger used anymore.

Signed-off-by: Jiro SEKIBA <jir@unicus.jp>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>