core: stop monitoring the 'usb' subsystem

Back in Linux < 3.6 days, the cdc-wdm ports exposed by the QMI driver
were flagged as owned by the 'usb' subsystem. That changed in 3.6 when
the subsystem was renamed to 'usbmisc':

  https://mail.gnome.org/archives/networkmanager-list/2012-June/msg00125.html

This patch removes all monitoring of the 'usb' subsystem completely,
which is anyway a valid subsystem but for which we shouldn't need any
special handling. Right now, with newer kernels, we were using that
monitoring exclusively to get notified of full USB device remove
events, which is really not required as we already process the port
removals one by one.

We simplify the logic everywhere that attempted to match either the
'usb' or 'usbmisc' subsystems, and we no longer require the explicit
checks for the port name being named 'cdc-wdm[0-9]*' in the code, as
that is already taken care of by the ID_MM_CANDIDATE udev tag rule.

plugins/simtech/mm-plugin-simtech.c

@@ -68,7 +68,7 @@ create_modem (MMPlugin *self,
 G_MODULE_EXPORT MMPlugin *
 mm_plugin_create (void)
 {
-    static const gchar *subsystems[] = { "tty", "net", "usb", NULL };
+    static const gchar *subsystems[] = { "tty", "net", "usbmisc", NULL };
     static const guint16 vendor_ids[] = { 0x1e0e, /* A-Link (for now) */
                                           0 };