settings: drop ibft settings plugin

The functionality of the ibft settings plugin is now handled by
nm-initrd-generator. There is no need for it anymore, drop it.

Note that ibft called iscsiadm, which requires CAP_SYS_ADMIN to work
([1]). We really want to drop this capability, so the current solution
of a settings plugin (as it is implemented) is wrong. The solution
instead is nm-initrd-generator.

Also, on Fedora the ibft was disabled and probably on most other
distributions as well. This was only used on RHEL.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1371201#c7
13 jobs for th/drop-ibft-settings-plugin in 64 minutes and 59 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #372277
allowed to fail
checkpatch

00:01:53

manual #372282
allowed to fail manual
t_centos:7.5.1804
passed #372283
t_centos:7.6.1810

00:23:01

passed #372286
t_debian:9

00:14:06

passed #372288
t_debian:sid

00:14:11

manual #372287
allowed to fail manual
t_debian:testing
passed #372278
t_fedora:28

00:22:23

passed #372279
t_fedora:29

00:27:15

passed #372280
t_fedora:30

00:24:19

manual #372281
allowed to fail manual
t_fedora:rawhide
passed #372284
t_ubuntu:16.04

01:04:58

passed #372285
t_ubuntu:18.04

00:24:23

 
  External
canceled NetworkManager test verification

05:05:00