Remove performance degrading joins from activity search (& api calls)

Like https://issues.civicrm.org/jira/browse/CRM-19815 but for activity

Note that removing the join on the SELECT & WHERE parts is relatively easy.

It is also possible to remove it on the ORDER BY (a less common function but one that happens if you sort the columns) and since that is more work to do I did some checks to see how much value it would give. The upshot is 'quite a bit'