f2fs: remove unused sbi->trigger_ssr_threshold

Commit a2a12b67 ("f2fs: export SSR allocation threshold") introduced
two threshold .min_ssr_sections and .trigger_ssr_threshold, but only
.min_ssr_sections is used, so just remove redundant one for cleanup.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/f2fs.h

@@ -1220,7 +1220,6 @@ struct f2fs_sb_info {
 	unsigned int total_valid_node_count;	/* valid node block count */
 	loff_t max_file_blocks;			/* max block index of file */
 	int dir_level;				/* directory level */
-	unsigned int trigger_ssr_threshold;	/* threshold to trigger ssr */
 	int readdir_ra;				/* readahead inode in readdir */
 
 	block_t user_block_count;		/* # of user blocks */