Remove unused "zeropad" entry from tree_list_entry

That was a hack, only needed because 'git fsck-objects' didn't look at
the raw tree format.  Now that fsck traverses the tree itself, we can
drop it.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>