btrfs: remove unused definition of tree_entry in extent-io-tree.c

The declaration was temporarily moved in a4055213 ("btrfs: unexport
all the temporary exports for extent-io-tree.c") and then should have
been removed in 6.0 in 071d19f5 ("btrfs: remove struct tree_entry in
extent-io-tree.c") but was not.  This was found by tool
https://github.com/jirislaby/clang-struct

 .

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>