t5613: drop test_valid_repo function
This function makes sure that "git fsck" does not report any
errors. But "--full" has been the default since
f29cd39
(fsck: default to "git fsck --full", 2009-10-20), and we can
use the exit code (instead of counting the lines) since
e2b4f63 (fsck: exit with non-zero status upon errors,
2007-03-05).
So we can just use "git fsck", which is shorter and more
flexible (e.g., we can use "git -C").
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>