Stop enabling event_cart on new installs

Although we have not fully extracted this, it seems that disabling does not
cause any errors, so it makes sense to stop enabling. Once we have been running
like this on our dev & demo sites for a while we can disable it
for any existing sites that do not have
event cart enabled

sql/civicrm_data/civicrm_extension.sqldata.php

@@ -20,10 +20,6 @@
       'full_name' => 'greenwich',
       'name' => 'Theme: Greenwich',
     ],
-    [
-      'full_name' => 'eventcart',
-      'name' => 'Event cart',
-    ],
     [
       'full_name' => 'financialacls',
       'name' => 'Financial ACLs',