FIX Don't assume a DataObject's table.

Knowing if a DataObject has a table or not is irrelevant - data is queried
out and pushed into solr all the same, whether there is a join table, a
base table, or any combination with maybe a few no-tables in between.
SomeDataClass::get() will fetch all data irrelevant of tables, that's the
point of an abstraction like SilverStripe's ORM.

src/Search/Indexes/SearchIndex.php

@@ -282,10 +282,6 @@ public function addClass($class, $options = array())
             throw new Exception('Can\'t add class to Index after fields have already been added');
         }
 
-        if (!DataObject::getSchema()->classHasTable($class)) {
-            throw new \InvalidArgumentException('Can\'t add classes which don\'t have data tables (no $db or $has_one set on the class)');
-        }
-
         $options = array_merge(array(
             'include_children' => true
         ), $options);