t/t7810-grep.sh: remove duplicate test_config()

t/t7810-grep.sh had its own test_config() function which served the
same purpose as the one in t/test-lib-functions.sh.  Removed, all tests
pass.

Signed-off-by: Jeremiah Mahler <jmmahler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7810-grep.sh

@@ -1096,11 +1096,6 @@ test_expect_success 'grep -E pattern with grep.patternType=fixed' '
 	test_cmp expected actual
 '
 
-test_config() {
-	git config "$1" "$2" &&
-	test_when_finished "git config --unset $1"
-}
-
 cat >expected <<EOF
 hello.c<RED>:<RESET>int main(int argc, const char **argv)
 hello.c<RED>-<RESET>{