Revert "If rev_content_model is null, use page_content_model."

Revert "If rev_content_model is null, use page_content_model."

This patch effectively reverted a5bc9f49cd01e49c, which
was the correct behavior. If a page were to have its content
model changed, the NULL revision rows would now be interpreted
as the page's new content model instead of the default content
model (which they should be).

This reverts commit 9a6ff0bd82faafbf9746a6fa3ed33f42092587c3.

Change-Id: I6801d09bd847f477222e503b912005b02a9df045