btrfs: remove struct tree_entry in extent-io-tree.c

This existed when we overloaded the tree manipulation functions for both
the extent_io_tree and the extent buffer tree.  However the extent
buffers are now stored in a radix tree, so we no longer need this
abstraction.  Remove struct tree_entry and use extent_state directly
instead.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>