t1300: remove duplicate test for `--file ../foo`

t1300: remove duplicate test for `--file ../foo`

We have two tests for checking that we can handle `git config --file
../other-config ...`. One, using `--file`, was introduced in 65807ee697
("builtin-config: Fix crash when using "-f <relative path>" from
non-root dir", 2010-01-26), then another, using `GIT_CONFIG`, came about
in 270a34438b ("config: stop using config_exclusive_filename",
2012-02-16).

The latter of these was then converted to use `--file` in f7e8714101
("t: prefer "git config --file" to GIT_CONFIG", 2014-03-20). Both where
then simplified in a5db0b77b9 ("t1300: extract and use
test_cmp_config()", 2018-10-21).

These two tests differ slightly in the order of the options used, but
other than that, they are identical. Let's drop one. As noted in
f7e8714101, we do still have a test for `GIT_CONFIG` and it shares the
implementation with `--file`.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>