selftest: af_unix: Remove test_unix_oob.c.

test_unix_oob.c does not fully cover AF_UNIX's MSG_OOB functionality,
thus there are discrepancies between TCP behaviour.

Also, the test uses fork() to create message producer, and it's not
easy to understand and add more test cases.

Let's remove test_unix_oob.c and rewrite a new test.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>