btrfs: remove write and wait of struct walk_control

The ->write and ->wait fields of struct walk_control, used for log trees,
are not used since 2008, more specifically since commit d0c803c4
("Btrfs: Record dirty pages tree-log pages in an extent_io tree") and
since commit d0c803c4 ("Btrfs: Record dirty pages tree-log pages in
an extent_io tree"). So just remove them, along with the function
btrfs_write_tree_block(), which is also not used anymore after removing
the ->write member.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>