- specfile: conflict systemd-bash-completion and systemd-mini-bash-completion
Otherwise the build system detects that systemd-bash-completion and
its mini variant are conflicting at files level even though those
packages can't be installed on the same system.

- specfile: clean up nss-* plugins descriptions and drop
nss-myhostname-config script for now.
Currently /etc/nsswitch.conf is supposed to be edited by the
sysadmin to enable the modules. However for some reasons only
nss-myhostname is removed from the conf file when the corresponding
package is uninstalled. This is inconsistent so let's remove it.
Actually I'm wondering if we shouldn't make those NSS plugins part
of the main package and get rid of all those sub-packages...

- specfile: remove old comments and unneeded sed command

- specfile: no need to create systemd-update-utmp-runlevel.service symlinks anymore
The symlinks in /usr/lib/systemd/system/<target>.target.wants/systemd-update-utmp-runlevel.service
are created in Makefile.am since commit d5d8429a12c4b1.
'reboot' and 'poweroff' targets initially had the symlinks but
there's not point since the latter conflicts shutdown.target whereas
the 2 targets pull it in.
See: https://github.com/systemd/systemd/pull/4429

- specfile: remove the following warnings:
[ 256s] warning: File listed twice: /usr/lib/systemd/system/dbus.target.wants
[ 256s] warning: File listed twice: /usr/lib/systemd/system/default.target.wants (forwarded request 440233 from fbui)