iface-modem-messaging: instead of mem1,mem2,mem3 use just a new default storage

There is no point in specifying a default 'mem1' memory storage, which is used
for reading/listing/deleting, as those are operations that need a specific
'mem1' set each time.

Also, there is no point in specifying separate default 'mem2' and 'mem3' memory
storages, specially because now we allow Sms.Store() to specify a storage.

So, we will now only have a 'default' memory storage, which is applicable for
both 'mem2' and 'mem3' (storing, sending from storage and deleting).

plugins/cinterion/mm-broadband-modem-cinterion.c

@@ -1163,7 +1163,7 @@ mm_broadband_modem_cinterion_new (const gchar *device,
                          MM_BASE_MODEM_PLUGIN, plugin,
                          MM_BASE_MODEM_VENDOR_ID, vendor_id,
                          MM_BASE_MODEM_PRODUCT_ID, product_id,
-                         MM_IFACE_MODEM_MESSAGING_SMS_MEM3_STORAGE, MM_SMS_STORAGE_MT,
+                         MM_IFACE_MODEM_MESSAGING_SMS_DEFAULT_STORAGE, MM_SMS_STORAGE_MT,
                          NULL);
 }