kernel: remove handling of xfrm[4|6]_mode_* modules

kernel: remove handling of xfrm[4|6]_mode_* modules

For kernel versions before 5.2, the required IPsec modes have to be
enabled explicitly (they are built-in for newer kernels).

Commit 1556ed155a9a ("kernel: mode_beet mode_transport mode_tunnel xfram
modules") tried to handle this, but it does not really work.

Since we don't support these kernel versions anymore and the code is
also broken, let's remove it.

Signed-off-by: Martin Schiller <ms@dev.tdt.de>
[Remove old generic config options too]
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>