GRMLBASE/39-modprobe: avoid usage of /lib/modprobe.d/50-nfs.conf

GRMLBASE/39-modprobe: avoid usage of /lib/modprobe.d/50-nfs.conf

The /lib/modprobe.d/50-nfs.conf file as shipped with
nfs-kernel-server >=1:2.6.2-1 causes plenty of problems,
like:

* https://bugs.debian.org/1024082 ("nfs-kernel-server:
  /lib/modprobe.d/50-nfs.conf causing problems with sysctl from busybox
  and with kmod")
* https://bugs.debian.org/1022172 ("/lib/modprobe.d/50-nfs.conf causes
  initramfs-tools to stop including sunrpc module for nfs")

So instead, let's ensure we don't ship the upstream file.

Closes: https://github.com/grml/grml/issues/193