Remove trivial implementations of getPagingColumn()

Summary:
Ref T7803. Some Query subclasses implement getPagingColumn() in a trivial way, usually to provide a table alias.

Formalize the concept of a primary table alias, and remove obsoleted getPagingColumn() implementations.

Test Plan: Issued affected queries.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7803

Differential Revision: https://secure.phabricator.com/D12356