Reimplement remove(3) without the lstat(2).

This assumes that it's more likely we're unlinking a file than a directory,
though even if that's not true, as long as a failed unlink(2) is cheaper
than a successful lstat(2) -- which seems likely since there's no data to
copy -- we still win.

Change-Id: I0210e9cd3d31b8cf1813c55c810262ef327382ed