f2fs: remove bulk remove_proc_entry() and unnecessary kobject_del()

Convert to use remove_proc_subtree() and kill kobject_del() directly.
kobject_put() actually covers kobject removal automatically, which is
single stage removal.

Signed-off-by: Yangtao Li <frank.li@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>