cli: drop NMC_RETURN

It's criminally ugly. Also -- totally useless.

These functions return NMCResultCode, call_cmd() expects them to do
so, and assigns the return_value itself.
15 jobs for lr/nmc-return in 42 minutes and 56 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Test
passed #457825
allowed to fail
checkpatch

00:01:13

manual #457830
allowed to fail manual
t_centos:7.5.1804
passed #457831
t_centos:7.6.1810

00:21:33

passed #457837
t_debian:10

00:20:47

passed #457836
t_debian:9

00:20:05

passed #457839
t_debian:sid

00:19:51

manual #457838
allowed to fail manual
t_debian:testing
passed #457826
t_fedora:28

00:30:57

passed #457827
t_fedora:29

00:41:46

passed #457828
t_fedora:30

00:26:52

manual #457829
allowed to fail manual
t_fedora:rawhide
passed #457832
t_ubuntu:16.04

00:33:33

passed #457833
t_ubuntu:18.04

00:33:52

manual #457835
allowed to fail manual
t_ubuntu:devel
manual #457834
allowed to fail manual
t_ubuntu:rolling