libnm: drop unused property setter NM_DEVICE_MANAGED

NM_DEVICE_MANAGED was intended to work like NM_DEVICE_AUTOCONNECT:
namely it would call the D-Bus property setter synchronously.

But such behavior is horrendous, we certainly don't want blocking calls
during a property getter.

Luckily this one instance was unused and never worked as the property
was marked as G_PARAM_READABLE. Just drop the setter.
17 jobs for th/libnm-dbus-rework-1 in 41 minutes and 5 seconds (queued for 18 seconds)
Status Job ID Name Coverage
  Test
passed #569310
allowed to fail
checkpatch

00:01:31

manual #569315
allowed to fail manual
t_centos:7.5.1804
passed #569316
t_centos:7.6.1810

00:27:25

passed #569322
t_debian:10

00:19:46

passed #569321
t_debian:9

00:18:01

passed #569324
t_debian:sid

00:21:10

manual #569323
allowed to fail manual
t_debian:testing
passed #569311
t_fedora:28

00:31:20

passed #569312
t_fedora:29

00:40:55

passed #569313
t_fedora:30

00:29:49

manual #569314
allowed to fail manual
t_fedora:rawhide
passed #569317
t_ubuntu:16.04

00:35:57

passed #569318
t_ubuntu:18.04

00:32:55

manual #569320
allowed to fail manual
t_ubuntu:devel
manual #569319
allowed to fail manual
t_ubuntu:rolling
 
  External
passed https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-code-upstream/673/

passed https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-code-upstream/673/

05:37:52