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>