Removed pqActiveView.

This class was lingering around since ancient times. We long replaced
this class with pqActiveObjects. Updated all code to directly use
pqActiveObjects.

Also updated MajorAPIChanges.md files to document this change.

Change-Id: I223ad816448ed4cd8f5d537af8400acd4a5cf237
Merge topic 'remove_pqActiveView' into next

74ff1014 Removed pqActiveView.