fsck: remove redundant date timestamp check

fsck: remove redundant date timestamp check

After calling "parse_timestamp(p, &end, 10)", we complain if "p == end",
which would imply that we did not see any digits at all. But we know
this cannot be the case, since we would have bailed already if we did
not see any digits, courtesy of extra checks added by 8e4309038f (fsck:
do not assume NUL-termination of buffers, 2023-01-19). Since then,
checking "p == end" is redundant and we can drop it.

This will make our lives a little easier as we refactor further.

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