t5305: move cleanup into test block

t5305: move cleanup into test block

We usually try to avoid doing any significant actions
outside of test blocks. Although "rm -rf" is unlikely to
either fail or to generate output, moving these to the
point of use makes it more clear that they are part of the
overall setup of "clone.git".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>