Provide a default "loadPage()" implementation on "CursorPagedPolicyAwareQuery"

Summary: Ref T13682. Many subclasses of "CursorPagedPolicyAwareQuery" have the same implementation of "loadPage()", and this is a sensible default behavior.

Test Plan: Examined changes to verify that all removed methods have the same behavior.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13682

Differential Revision: https://secure.phabricator.com/D21838