t0066: drop setup of "dir5"

t0066: drop setup of "dir5"

The symlink setup in t0066 makes several directories with links, dir4
through dir6. But ever since dir5 was introduced in fa1da7d2ee
(dir-iterator: add flags parameter to dir_iterator_begin, 2019-07-10),
it has never actually been used. It was left over from an earlier
iteration of the patch which tried to handle recursive symlinks
specially, as seen in:

  https://lore.kernel.org/git/20190502144829.4394-7-matheus.bernardino@usp.br/

It's not hurting any of the existing tests to be there, but the extra
setup is confusing to anybody trying to read and understand the tests.
Let's drop the extra directory, and we'll rename "dir6" to "dir5" so
nobody wonders whether the gap in naming is important.

Helped-by: Matheus Tavares Bernardino <matheus.tavb@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>