quota: simplify remove_inode_dquot_ref()

Simplify the remove_inode_dquot_ref() to make it more obvious
that now we keep one reference for each dquot from inodes.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Signed-off-by: Jan Kara <jack@suse.cz>