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

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>