mktag tests: don't redirect stderr to a file needlessly

Remove the redirection of stderr to "message" in the valid tag test. This pattern seems to have been copy/pasted from the failure case in 446c6faec6 (New tests and en-passant modifications to mktag., 2006-07-29). While I'm at it do the same for the "replace" tests. The tag creation I'm changing here seems to have been copy/pasted from the "mktag" tests to those tests in cc400f50112 (mktag: call "check_sha1_signature" with the replacement sha1, 2009-01-23). Nobody examines the contents of the resulting "message" file, so the net result is that error messages cannot be seen in "sh t3800-mktag.sh -v" output. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>