GUnixConnection: Remove comment about Linux

GUnixConnection: Remove comment about Linux

Since the previous commit, the g_unix_connection_send_credentials() /
g_unix_connection_receive_credentials() functions now also works on
FreeBSD since GUnixCredentialsMessage now works there.

The main idea is that the g_unix_connection_send_credentials() /
g_unix_connection_receive_credentials() functions are the "main" API
for getting credentials (one way or the other). So it's better to
avoid advertising where it is currently implemented.

Signed-off-by: David Zeuthen <davidz@redhat.com>