`config.s390x` had entries already present in `config`.

When generating the config used by vmtest, we concatenate the `config`
file with the `config.{arch}` one, making those entries duplicated.

This patch removes that duplication.

Before:
$ comm -1 -2  <(sort tools/testing/selftests/bpf/config.s390x) <(sort
tools/testing/selftests/bpf/config)
CONFIG_MODULE_SIG=y
CONFIG_MODULES=y
CONFIG_MODULE_UNLOAD=y
$

Ater:
$ comm -1 -2  <(sort tools/testing/selftests/bpf/config.s390x) <(sort
tools/testing/selftests/bpf/config)
$
Signed-off-by: Manu Bretelle <chantr4@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221021210701.728135-2-chantr4@gmail.com