MDL-60434 analytics: Remove legacy is_valid method

This method was used when the API was tied to students at risk model,
this method does not make sense any more as it is up to each target
to define what is a valid course.

analytics/classes/course.php

@@ -365,20 +365,6 @@ class course implements \core_analytics\analysable {
         return $this->course;
     }
 
-    /**
-     * Is the course valid to extract indicators from it?
-     *
-     * @return bool
-     */
-    public function is_valid() {
-
-        if (!$this->was_started() || !$this->is_finished()) {
-            return false;
-        }
-
-        return true;
-    }
-
     /**
      * Has the course started?
      *