t0021: remove debugging cruft

The redirection of the standard error stream to a temporary file is
a leftover cruft during debugging.  Remove it.

Besides, it is reported by folks on the Windows that the test is
flaky with this redirection; somebody gets confused and this
merely-redirected-to file gets marked as delete-pending by git.exe
and makes it finish with a non-zero exit status when "git checkout"
finishes.  Windows folks may want to figure that one out, but for
the purpose of this test, it shouldn't become a show-stopper.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0021-conversion.sh

@@ -27,8 +27,7 @@ file_size () {
 
 filter_git () {
 	rm -f rot13-filter.log &&
-	git "$@" 2>git-stderr.log &&
-	rm -f git-stderr.log
+	git "$@"
 }
 
 # Compare two files and ensure that `clean` and `smudge` respectively are