networkAgent: Drop VPN plugin cache

libnm doesn't only search for plugins in the regular VPN plugin directory,
but also in the legacy location and the directory pointed to by the
NM_VPN_PLUGIN_DIR environment variable (if set).

We don't monitor the additional directories, so it's possible for our cache
to become outdated.

Instead of trying to play catch-up with libnm's internals, do what nm-applet
does and use the appropriate API to look up the plugin on each request.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/2386

js/misc/config.js.in

@@ -15,6 +15,5 @@ var LOCALEDIR = '@datadir@/locale';
 /* other standard directories */
 var LIBEXECDIR = '@libexecdir@';
 var PKGDATADIR = '@datadir@/@PACKAGE_NAME@';
-var VPNDIR = '@vpndir@';
 /* g-i package versions */
 var LIBMUTTER_API_VERSION = '@LIBMUTTER_API_VERSION@'

js/misc/meson.build

@@ -7,7 +7,6 @@ jsconf.set10('HAVE_BLUETOOTH', bt_dep.found())
 jsconf.set10('HAVE_NETWORKMANAGER', have_networkmanager)
 jsconf.set('datadir', datadir)
 jsconf.set('libexecdir', libexecdir)
-jsconf.set('vpndir', vpndir)
 
 config_js = configure_file(
   input: 'config.js.in',