Deprecate ability to disable search pages (no reason for it and causes problems and code complexity)

Problem/Motivation

On #2664830: Add search capability to help topics → , andypost/ambermatz discovered that if you go to admin/search/pages and disable the Help Search page, then any page (such as admin/help) that is displaying the Help Search block will have a WSOD.

The reason is that the Search Block Form doesn't verify that the page it is supposed to submit to exists and is active. It should.

Proposed resolution

Alternative 1:
A patch was proposed by jhodgdon on #2664830-203: Add search capability to help topics → -- here's the interdiff:
https://www.drupal.org/files/issues/2019-10-08/interdiff_13.txt →

It needs a test. Also the part for the doc block in the interface is on another issue. Only the SearchBlockForm needs the patch.

Alternative 2:
Another way to resolve this would be to remove the ability to disable search pages. It is not clear why you would want to disable them anyway.

Remaining tasks

1. Decide which way to go -- the consensus is that we should remove the ability to disable search pages.

2. Make a patch.

User interface changes

The ability to disable search pages will not exist any more. You will only be able to add and delete pages, not disable them.

API changes

Functions related to disabling search pages and checking status will be removed/deprecated.

Data model changes

The status component of search page plugin configuration will be removed.

Release notes snippet

TBD