BCH_INODE_i_size_dirty dates from before we had logged operations for
truncate (as well as finsert) - it hasn't been needed since before
bcachefs was mainlined.

BCH_INODE_i_sectors_dirty hasn't been needed since we started always
updating i_sectors transactionally - it's been unused for even longer.

BCH_INODE_backptr_untrusted also hasn't been used since prior to
mainlining; when unlinking a hardling, we zero out the backpointer
fields if they're for the dirent being removed.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>