btrfs: remove unused parameter from readahead_tree_block

The parent_transid parameter has been unused since its introduction in
ca7a79ad8dbe2466 ("Pass down the expected generation number when reading
tree blocks").  In reada_tree_block, it was even wrongly set to leafsize.
Transid check is done in the proper read and readahead ignores errors.

Signed-off-by: David Sterba <dsterba@suse.cz>