selftests/bpf: Remove entries from config.s390x already present in config

`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 <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/config.s390x

···
 CONFIG_MEMCG=y
 CONFIG_MEMORY_HOTPLUG=y
 CONFIG_MEMORY_HOTREMOVE=y
-CONFIG_MODULE_SIG=y
-CONFIG_MODULE_UNLOAD=y
-CONFIG_MODULES=y
 CONFIG_NAMESPACES=y
 CONFIG_NET=y
 CONFIG_NET_9P=y