btrfs: remove NULL transaction support for btrfs_lookup_extent_info()

There are no callers of btrfs_lookup_extent_info() that pass a NULL value
for the transaction handle argument, so there's no point in having special
logic to deal with the NULL. The last caller that passed a NULL value was
removed in commit 19b546d7

 ("btrfs: relocation:
Use btrfs_find_all_leafs to locate data extent parent tree leaves").

So remove the NULL handling from btrfs_lookup_extent_info().

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