Remove ensure associations

[afonic]

@ryanmitchell I think this was the other culprit for all the slowness.

The TermRepository was calling $this->ensureAssociations(); which populates the associations index :

protected function ensureAssociations()
{
    Taxonomy::all()->each(function ($taxonomy) {
        $this->store->store($taxonomy->handle())->index('associations');
    });
}

This store is quite complex (over 10MB in my case) and it's probably not required when using the eloquent driver.

I also added a test to make sure the Terms facade still gets the correct count.

PS. With this change and your other PR the first page load is under 1 second.