jbd2: revert "jbd2: add COW fields to struct jbd2_journal_handle"

This reverts commit 93737456.

The cow-snapshots effort is no longer active, so remove these extra
fields to shrink down the handle structure.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>