all: goodbye libnm-glib

This removes libnm-glib, libnm-glib-vpn, and libnm-util for good.
The it has been replaced with libnm since NetworkManager 1.0, disabled
by default since 1.12 and no up-to-date distributions ship it for years
now.

Removing the libraries allows us to:

* Remove the horrible hacks that were in place to deal with accidental use
  of both the new and old library in a single process.
* Relief the translators of maintenance burden of similar yet different
  strings.
* Get rid of known bad code without chances of ever getting fixed
  (libnm-glib/nm-object.c and libnm-glib/nm-object-cache.c)
* Generally lower the footprint of the releases and our workspace

If there are some really really legacy users; they can just build
libnm-glib and friends from the NetworkManager-1.16 distribution. The
D-Bus API is stable and old libnm-glib will keep working forever.

https://github.com/NetworkManager/NetworkManager/pull/308

contrib/scripts/nm-ci-run.sh

@@ -100,7 +100,6 @@ run_autotools() {
             \
             --with-ebpf=no \
             \
-            --with-libnm-glib=yes \
             --with-iwd=yes \
             --with-ofono=yes \
             --enable-teamdctl=$_WITH_LIBTEAM \
@@ -171,7 +170,6 @@ run_meson() {
         \
         -D ebpf=false \
         \
-        -D libnm_glib=true \
         -D iwd=true \
         -D ofono=true \
         -D teamdctl=$_WITH_LIBTEAM \