Add ability to remove a parent theme's page templates

[billerickson]

When building a website using a child theme, I'll often want to remove page templates that the parent theme provides but aren't useful/relevant to the specific site I'm building.

You can create a page template with the same name to override the parent theme's, but you can't simply remove it.

This could be a use case of #13265, or we could add function that leverages #13265

[obenland]

This was addressed in r27297 and r27470, with the theme_page_templates filter.