Eliminate post-processor cache

[westonruter]

Feature description

It has become clear that the post-processor caching is not working. It is currently only offered when the user has an external object cache, and when it is available then very frequently users will see this notice appear on the AMP settings screen:

The reality is that pages with highly-variable content are extremely common, whether that be random numbers, timestamps, or nonces. Therefore I believe this is a feature that should be removed since it is not effective.

We may also want to eliminate support for sending 304 Not Modified responses (#1481). Even though this doesn't directly cause data to be put in to a cache, it can be wasted processing time to compute the ETag if it changes with each request.

PRs related to post-processor response caching:

• Issue 959: cache post processor response #1156 for Add caching of post processor output #959
• Improve post processor response cache #1325 for Twenty Seventeen's use of uniqid() breaks post-processor response cache #1239

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[pierlon]

@westonruter Would you consider removing post-processor related cache as well, or let them expire?

[westonruter]

Removing the cache entries? There wouldn't be a way to iterate over all of them, so no, there's nothing we can do to delete them. They'll be deleted when they expire.

[pierlon]

Yep that's true... I see no problem with them expiring anyways.

[pierlon]

The wiki page on post-processor cache may need to be updated to indicate it has been removed.