contrib/rpm: remove NetworkManager-devel package

contrib/rpm: remove NetworkManager-devel package

NetworkManager-devel package contained development headers that
are useful without libnm-glib and without glib. But it is also
based on the legacy libnm-glib library as it has headers like
"/usr/include/NetworkManager/NetworkManager.h".

A glib-free devel package based on the new libnm library would
be needed to provide "/usr/include/libnm/nm-dbus-interface.h".

But that would amount to 4 devel packages. Instead, just move
the content of NetworkManager-devel into NetworkManager-glib-devel
package.

Note that NetworkManager-devel already contained several truely
libnm-glib dependent files, like the vala bindings (which require
libnm-glib). So that was another bug in the packaging and is fixed
by moving it all to NetworkManager-glib-devel.

https://bugzilla.gnome.org/show_bug.cgi?id=755938