btrfs: remove check for impossible block start for an extent map at fiemap

During fiemap we are testing if an extent map has a block start with a
value of EXTENT_MAP_LAST_BYTE, but that is never set on an extent map,
and never was according to git history. So remove that useless check.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
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>