crawl: deprecate use of BeesCrawl to search the extent tree

crawl: deprecate use of BeesCrawl to search the extent tree

BeesScanModeExtent can do that by itself now.  Overloading the subvol
crawl code resulted in an ugly, inefficient hack, and we definitely
don't want to accidentally continue to use it.

Remove the support for reading the extent tree and add some `assert`s
to make sure it isn't still used somewhere.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>