CRM_Activity_Tokens - Simplify prefetch. Remove special-cases for `actionSearchResult`.

CRM_Activity_Tokens - Simplify prefetch. Remove special-cases for `actionSearchResult`.

Before: There are two distinct ways in which `CRM_Activity_Tokens` can be called, eg

* For scheduled reminders, it's given `$row->context[actionSearchResult]`. The query is
  inspected for data.
* For everything else, it's given `$row->context[activityId]`. The value is

After: `CRM_Activity_Tokens` always receives activities as `$row->context[activityId]`.

Comment: There are pre-existing tests for activity-based reminders and tokens in
`CRM_Core_BAO_ActionScheduleTest` and `CRM_Activity_ActionMappingTest`.