btrfs: delete pointless BUG_ONs on extent item size

Checking extent item size in add_inline_refs() is redundant, we do that
already in tree-checker after reading the extent buffer and it won't
change under normal circumstances.  It was added long ago in
8da6d581 ("Btrfs: added btrfs_find_all_roots()") and does not seem
to have a clear purpose.

Similar case in extent_from_logical(), added in a542ad1b

 ("btrfs:
added helper functions to iterate backrefs").

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