t4018,7810,7811: remove test_config() redefinition

test_config() is already a well-defined function in
test-lib-functions.sh.  Don't duplicate it unnecessarily.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4018-diff-funcname.sh

@@ -93,11 +93,6 @@ sed -e '
 	s/song;/song();/
 ' <Beer.perl >Beer-correct.perl
 
-test_config () {
-	git config "$1" "$2" &&
-	test_when_finished "git config --unset $1"
-}
-
 test_expect_funcname () {
 	lang=${2-java}
 	test_expect_code 1 git diff --no-index -U1 \